Category Archives: Firefox

Faster Firefox startup & shutdown with add-ons present

Thanks to some recent improvements by Kris Maglione, add-ons built with Firefox’s add-ons SDK are now much faster to start up and shut down. For users with multiple add-ons installed, this can make a large difference. For example, one user reported the following.

I’ve noticed a dramatic improvement in startup and shutdown time since the fixes landed on aurora.

Configuration: Two windows with 15 tabs each and 22 addons.

  • Startup: 25 seconds -> 14 seconds (44% reduction)
  • Shutdown: 30 seconds -> 5 seconds (83% reduction)

Shutdown used to take so long it was triggering a minidump.

Your mileage may vary, but these improvements have the potential to benefit many users. They are present in Firefox 50 (currently in Beta, and due for release on November 15) and all later versions.

How to get localized Firefox Nightly builds

One of the easiest and best ways that someone can help Mozilla and Firefox is to run Firefox Nightly. I’ve been doing it on my Windows, Mac and Linux machines for the past couple of months. It requires daily restarts, but otherwise it has been a smooth experience for me.

Unfortunately the number of Nightly users has been steadily dropping for some time, which hurts our ability to catch crashes and other regressions early. Pascal Chevrel and Marcia Knous are leading efforts underway to reverse this trend.

One problem with Nightly builds has been their visibility. In particular, finding localized (non-English) builds was difficult. That situation has just improved: thanks to Kohei Yoshino there is now a single page containing Nightly builds for all platforms and locales. As far as I know there are no other pages that currently link to that page, but perhaps that will happen as part of the planned work to give Nightly builds a place on mozilla.org.

If you have friends and family who would like to help Mozilla and are willing to use pre-release versions of Firefox, please suggest Firefox Nightly to them.

How to switch to a 64-bit Firefox on Windows

I recently wrote about 64-bit Firefox builds on Windows, explaining why you might want to switch — it can reduce the likelihood of out-of-memory crashes — and also some caveats.

However, I didn’t explain how to switch, so I will do that now.

First, if you want to make sure that you aren’t already running a 64-bit Firefox, type “about:support” in the address bar and then look at the User Agent field in the Application Basics table near the top of the page.

  • If it contains the string “Win64”, you are already running a 64-bit Firefox.
  • If it contains the string “WOW64“, you are running a 32-bit Firefox on a 64-bit Windows installation, which means you can switch to a 64-bit build.
  • Otherwise, you are running a 32-bit Firefox on a 32-bit Windows installation, and cannot switch to a 64-bit Firefox.

Here are links to pages contain 64-bit builds for all the different release channels.

  • Release
  • Beta
  • Developer Edition
  • Nightly:  This is a user-friendly page, but it only has the en-US locale.
  • Nightly: This is a more intimidating page, but it has all locales. Look for a file with a name of the form firefox-<VERSION>.<LOCALE>.win64.installer.exe, e.g. firefox-50.0a1.de.win64.installer.exe for Nightly 50 in German.

By default, 32-bit Firefox and 64-bit Firefox are installed to different locations:

  • C:\Program Files (x86)\Mozilla Firefox\
  • C:\Program Files\Mozilla Firefox\

If you are using a 32-bit Firefox and then you download and install a 64-bit Firefox, by default you will end up with two versions of  Firefox installed. (But note that if you let the 64-bit Firefox installer add shortcuts to the desktop and/or taskbar, these shortcuts will replace any existing shortcuts to 32-bit Firefox.)

Both the 32-bit Firefox and the 64-bit Firefox will use the same profile, which means all your history, bookmarks, extensions, etc., will be available in either version. You’ll be able to run both versions, though not at the same time with the same profile. If you decide you don’t need both versions you can simply remove the unneeded version through the Windows system settings, as normal; your profile will not be touched when you do this.

Finally, there is a plan to gradually roll out 64-bit Firefox to Windows users in increasing numbers.

Firefox 64-bit for Windows can take advantage of more memory

By default, on Windows, Firefox is a 32-bit application. This means that it is limited to using at most 4 GiB of memory, even on machines that have more than 4 GiB of physical memory (RAM). In fact, depending on the OS configuration, the limit may be as low as 2 GiB.

Now, 2–4 GiB might sound like a lot of memory, but it’s not that unusual for power users to use that much. This includes:

  • users with many (dozens or even hundreds) of tabs open;
  • users with many (dozens) of extensions;
  • users of memory-hungry web sites and web apps; and
  • users who do all of the above!

Furthermore, in practice it’s not possible to totally fill up this available space because fragmentation inevitably occurs. For example, Firefox might need to make a 10 MiB allocation and there might be more than 10 MiB of unused memory, but if that available memory is divided into many pieces all of which are smaller than 10 MiB, then the allocation will fail.

When an allocation does fail, Firefox can sometimes handle it gracefully. But often this isn’t possible, in which case Firefox will abort. Although this is a controlled abort, the effect for the user is basically identical to an uncontrolled crash, and they’ll have to restart Firefox. A significant fraction of Firefox crashes/aborts are due to this problem, known as address space exhaustion.

Fortunately, there is a solution to this problem available to anyone using a 64-bit version of Windows: use a 64-bit version of Firefox. Now, 64-bit applications typically use more memory than 32-bit applications. This is because pointers, a common data type, are twice as big; a rough estimate for 64-bit Firefox is that it might use 25% more memory. However, 64-bit applications also have a much larger address space, which means they can access vast amounts of physical memory, and address space exhaustion is all but impossible. (In this way, switching from a 32-bit version of an application to a 64-bit version is the closest you can get to downloading more RAM!)

Therefore, if you have a machine with 4 GiB or less of RAM, switching to 64-bit Firefox probably won’t help. But if you have 8 GiB or more, switching to 64-bit Firefox probably will help the memory usage situation.

Official 64-bit versions of Firefox have been available since December 2015. If the above discussion has interested you, please try them out. But note the following caveats.

  • Flash and Silverlight are the only supported 64-bit plugins.
  • There are some Flash content regressions due to our NPAPI sandbox (for content that uses advanced features like GPU acceleration or microphone APIs).

On the flip side, as well as avoiding address space exhaustion problems, a security feature known as ASLR works much better in 64-bit applications than in 32-bit applications, so 64-bit Firefox will be slightly more secure.

Work is being ongoing to fix or minimize the mentioned caveats, and it is expected that 64-bit Firefox will be rolled out in increasing numbers in the not-too-distant future.

UPDATE: Chris Peterson gave me the following measurements about daily active users on Windows.

  • 66.0% are running 32-bit Firefox on 64-bit Windows. These users could switch to a 64-bit Firefox.
  • 32.3% are running 32-bit Firefox on 32-bit Windows. These users cannot switch to a 64-bit Firefox.
  • 1.7% are running 64-bit Firefox already.

UPDATE 2: Also from Chris Peterson, here are links to 64-bit builds for all the channels:

More compacting GC

Jon Coppeard recently extended SpiderMonkey’s compacting GC abilities. Previously, the GC could only compact GC arena containing JavaScript objects. Now it can also compact arenas containing shapes (a data structure used within SpiderMonkey which isn’t visible to user code) and strings, which are two of the largest users of memory in the GC heap after objects.

These improvements should result in savings of multiple MiBs in most workloads, and they are on track to ship in Firefox 48, which will be released in early August. Great work, Jon!

I rewrote Firefox’s BMP decoder

Recently I’ve been deliberately working on some areas of Firefox I’m unfamiliar with, particular relating to graphics. This led me to rewriting Firefox’s BMP decoder and learn a number of interesting things along the way.

Image decoding

Image decoding is basically the process of taking an image encoded in a file and extracting its pixels. In principle it’s simple. You start by reading some information about the image, such as its size and colour depth, which typically comes in some kind of fixed-size header. Then you read the pixel data, which is variable-sized.

This isn’t hard if you have all the data available at the start. But in the context of a browser it makes sense to decode incrementally as data comes in over the network. In that situation you have to be careful and constantly check if you have enough data yet to safely read the next chunk of data. This checking is error-prone and tends to spread itself all over the image decoder.

For this reason, Seth Fowler recently wrote a new class called StreamingLexer that encapsulates this checking and exposes a nice state-based interface to image decoders. When a decoder changes state (e.g. it finishes reading the header) it tells StreamingLexer how many bytes it needs to safely enter the next state (e.g. to read the first row of pixels) and StreamingLexer won’t return control to the decoder until that many bytes are available.

Another consideration when decoding images is that you can’t trust them. E.g. an image might claim to be 100 x 100 pixels but actually contain less data than that. If you’re not careful you could easily read memory you shouldn’t, which could cause crashes or security problems. StreamingLexer helps with this, too.

StreamingLexer makes image decoders simpler and safer, and converting the BMP decoder to use it was my starting point.

The BMP format

The BMP format comes from Windows. On the web it’s mostly used on the web for favicons though it can be used for normal images.

There’s no specification for BMP. There are eight in-use versions of the format that I know of, with later versions mostly(!) extending earlier versions. If you’re interested, you can read the brief description of all these versions that I wrote in a big comment at the top of nsBMPDecoder.cpp.

Because the format is so gnarly I started getting nervous that my rewrite might  introduce bugs in some of the darker corners, especially once Seth told me that our BMP test coverage wasn’t that good.

So I searched around and found Jason Summers’ wonderful BMP Suite, which exercises pretty much every corner of the BMP format. Version 2.3 of the BMP Suite contains 57 images, 23 of which are “good” (obviously valid), 14 of which are “bad” (obviously invalid) and 20 of which are “questionable” (not obviously valid or invalid). The presence of this last category demonstrates just how ill-specified BMP is as a format, and some of the “questionable” tests have two or three reference images, any of which could be considered a correct rendering. (Furthermore, it’s possible to render a number of the “bad” images in a reasonable way.)

This test suite was enormously helpful. As well as giving me greater confidence in my changes, it immediately showed that we had several defects in the existing BMP decoder, particular relating to the scaling of 16-bit colors and an almost complete lack of transparency handling. In comparison, Chrome rendered pretty much all the images in BMP suite reasonably, and Safari and Edge got a few wrong but still did better than Firefox.

Fixing the problems

So I fixed these problems as part of my rewrite. The following images show a number of test images that Firefox used to render incorrectly; in each case a correct rendering is on the left, and our old incorrect rendering is on the right.

bad-bmp-2 bad-bmp-3 bad-bmp-4 bad-bmp-5

It’s clear that the old defects were mostly related to colour-handling, though the first pair of images shows a problem relating to the starting point of the pixel data.

(These images are actually from an old version of Firefox with version 2.4 of BMP Suite, which I just discovered was released only a few days ago. I just filed a bug to update the copy we use in automated testing. Happily, it looks like the new code does reasonable things with all the images added in v2.4.)

These improvements will ship in Firefox 44, which is scheduled to be released in late January, 2016. And with that done I now need to start thinking about rewriting the GIF decoder

moz-icon: a curious corner of Firefox

Here’s an odd little feature in Firefox. Enter the following text into the address bar.

 moz-icon://.pdf?size=128

On my Linux box, it shows the following icon image.

pdf-icon-linux

On my Windows laptop, it shows a different icon image.

pdf-icon-windows

On my Mac Laptop, that URL doesn’t work but if I change the “128” to “64” it shows this icon image.

pdf-icon-mac

In each case we get the operating system’s icon for a PDF file.

Change the “size” (up to a maximum of 255) value and you’ll get a different size. Except on Mac the limit seems to be lower, probably due to the retina screen in some way.

Change the file extension and you’ll get a different icon. You can try all sorts, like “.xml”, “.cpp”, “.js”, “.py”, “.doc”, etc.

How interesting. So what’s this for? As far as I understand, it’s used to make local directory listing pages look nice. E.g. on my Linux box if I type “file:///home/njn/” into the address bar I get a listing of my home directory, part of which looks like the following.

home-directory-listing

That listing uses this mechanism to show the appropriate system icon for each file type.

Furthermore, this feature is usable from regular web pages! Just put a “moz-icon” URL into an <image> tag and you’ll get OS-specific icons in your webpage.

That’ll only work on Firefox, though. Chrome actually has a similar mechanism, though it’s not usable from regular web pages. (For more detail — much more — read here.) As far as I know Safari, IE and Edge do not have such a mechanism; I’m not sure if they support listing of local directories in this fashion.

A work-around for Tree Style Tab breakage on Firefox Nightly caused by mozRequestAnimationFrame removal

This post is aimed at Firefox Nightly users who also use the Tree Style Tab extension. Bug 909154 landed last week. It removed support for the prefixed mozRequestionAnimationFrame function, and broke Tree Style Tab. The GitHub repository that hosts Tree Style Tab’s code has been updated, but that has not yet made it into the latest Tree Style Tab build, which has version number 0.15.2015061300a003855.

Fortunately, it’s fairly easy to modify your installed version of Tree Style Tab to fix this problem. (“Fairly easy”, at least, for the technically-minded users who run Firefox Nightly.)

  • Find the Tree Style Tabs .xpi file. On my Linux machine, it’s at ~/.mozilla/firefox/ndbcibpq.default-1416274259667/extensions/treestyletab@piro.sakura.ne.jp.xpi. Your profile name will not be exactly the same. (In general, you can find your profile with these instructions.)
  • That file is a zip file. Edit the modules/lib/animationManager.js file within that file, and change the two occurrences of mozRequestAnimationFrame to requestAnimationFrame. Save the change.

I did the editing in vim, which was easy because vim has the ability to edit zip files in place. If your editor does not support that, it might work if you unzip the code, edit the file directly, and then rezip, but I haven’t tried that myself. Good luck.

Compacting GC

Go read Jon Coppeard’s description of the compacting GC algorithm now used by SpiderMonkey!

Firefox 41 will use less memory when running AdBlock Plus

Last year I wrote about AdBlock Plus’s effect on Firefox’s memory usage. The most important part was the following.

First, there’s a constant overhead just from enabling ABP of something like 60–70 MiB. […] This appears to be mostly due to additional JavaScript memory usage, though there’s also some due to extra layout memory.

Second, there’s an overhead of about 4 MiB per iframe, which is mostly due to ABP injecting a giant stylesheet into every iframe. Many pages have multiple iframes, so this can add up quickly. For example, if I load TechCrunch and roll over the social buttons on every story […], without ABP, Firefox uses about 194 MiB of physical memory. With ABP, that number more than doubles, to 417 MiB.

An even more extreme example is this page, which contains over 400 iframes. Without ABP, Firefox uses about 370 MiB. With ABP, that number jumps to 1960 MiB.

(This description was imprecise; the overhead is actually per document, which includes both top-level documents in a tab and documents in iframes.)

Last week Mozilla developer Cameron McCormack landed patches to fix bug 77999, which was filed more than 14 years ago. These patches enable sharing of CSS-related data — more specifically, they add data structures that share the results of cascading user agent style sheets — and in doing so they entirely fix the second issue, which is the more important of the two.

For example, on the above-mentioned “extreme example” (a.k.a. the Vim Color Scheme Test) memory usage dropped by 3.62 MiB per document. There are 429 documents on that page, which is a total reduction of about 1,550 MiB, reducing memory usage for that page down to about 450 MiB, which is not that much more than when AdBlock Plus is absent. (All these measurements are on a 64-bit build.)

I also did measurements on various other sites and confirmed the consistent saving of ~3.6 MiB per document when AdBlock Plus is enabled. The number of documents varies widely from page to page, so the exact effect depends greatly on workload. (I wanted to test TechCrunch again, but its front page has been significantly changed so it no longer triggers such high memory usage.) For example, for one of my measurements I tried opening the front page and four articles from each of nytimes.com, cnn.com and bbc.co.uk, for a total of 15 tabs. With Cameron’s patches applied Firefox with AdBlock Plus used about 90 MiB less physical memory, which is a reduction of over 10%.

Even when AdBlock Plus is not enabled this change has a moderate benefit. For example, in the Vim Color Scheme Test the memory usage for each document dropped by 0.09 MiB, reducing memory usage by about 40 MiB.

If you want to test this change out yourself, you’ll need a Nightly build of Firefox and a development build of AdBlock Plus. (Older versions of AdBlock Plus don’t work with Nightly due to a recent regression related to JavaScript parsing). In Firefox’s about:memory page you’ll see the reduction in the “style-sets” measurements. You’ll also see a new entry under “layout/rule-processor-cache”, which is the measurement of the newly shared data; it’s usually just a few MiB.

This improvement is on track to make it into Firefox 41, which is scheduled for release on September 22, 2015. For users on other release channels, Firefox 41 Beta is scheduled for release on August 11, and Firefox 41 Developer Edition is scheduled to be released in the next day or two.