Category Archives: Uptime

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

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.

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:

I want more users on the Nightly channel

I have been working recently on a new Platform Engineering initiative called Uptime, the goal of which is to reduce Firefox’s crash rate on both desktop and mobile. As a result I’ve been spending a lot of time looking at crash reports, particular on the Nightly channel. This in turn has increased my appreciation of how important Nightly channel users are.

A crash report from a Nightly user is much more useful than a crash report from a non-Nightly user, for two reasons.

  • If a developer lands a change that triggers crashes for Nightly users, they will get fast feedback via crash reports, often within a day or two.  This maximizes the likelihood of a fix, because the particular change will be fresh in the developer’s mind. Also, backing out changes is usually easy at this point. In contrast, finding out about a crash weeks or months later is less useful.
  • Because a new Nightly build is done every night, if a new crash signature appears, we have a fairly small regression window. This makes it easier to identify which change caused the new crashes.

Also, Nightly builds contain some extra diagnostics and checks that can also be helpful with identifying a range of problems. (See MOZ_DIAGNOSTIC_ASSERT for one example.)

If we could significantly increase the size of our Nightly user population, that would definitely help reduce crash rates. We would get data about a wider range of crashes. We would also get stronger signals for specific crash-causing defects. This is important because the number of crash reports received for each Nightly build is relatively low, and it’s often the case that a cluster of crash reports that come from two or more different users will receive more attention than a cluster that comes from a single user.

(You might be wondering how we distinguish those two cases. Each crash report doesn’t contain enough information to individually identify the user — unless the user entered their email address into the crash reporting form — but crash reports do contain enough information that you can usually tell if two different crash reports have come from two different users. For example, the installation time alone is usually enough, because it’s measured to the nearest second.)

All this is doubly true on Android, where the number of Nightly users is much smaller than on Windows, Mac and Linux.

Using the Nightly channel is not the best choice for everyone. There are some disadvantages.

  • Nightly is less stable than later channels, but not drastically so. The crash rate is typically 1.5–2.5 times higher than Beta or Release, though occasionally it spikes higher for a short period. So a Nightly user should be comfortable with the prospect of less stability.
  • Nightly gets updated every 24 hours, which some people would find annoying.

There are also advantages.

  • Nightly users get to experience new features and fixes immediately.
  • Nightly users get the satisfaction that they are helping produce a better Firefox. The frustration of any crash is offset by the knowledge that the information in the corresponding crash report is disproportionately valuable. Indeed, there’s a non-trivial likelihood that a single crash report from a Nightly user will receive individual attention from an engineer.

If you, or somebody you know, thinks that those advantages outweigh the disadvantages, please consider switching. Thank you.