Firefox 6 breaking some themes when viewing plugin content

A number of bug reports have surfaced after the Firefox 6 update, where users are getting transparent or partially transparent content in their plugins, like Flash and the Adobe PDF viewer. These bugs are caused by a breaking change that was introduced in the default Firefox theme for Windows (Aero only). The Add-ons Team wasn’t aware of this change and didn’t discover it on time. We apologize for the breakage.

The bug that introduced this change has a good explanation by Jim Mathies on what changed and how to fix it, which I will just quote verbatim since I couldn’t explain it any better:

In past versions Firefox calculated the location of glass boundaries (the point at which transparent window margins end) automatically. In version 6.0 this was replaced with an opt-out style where XUL elements are marked explicitly as opaque via css. Currently various UI elements have this new style including the main content area of the browser, downloads window content, and extension’s about dialog.

The css property we use to identify opaque regions is through a new value for the moz-appearance style:

-moz-appearance: -moz-win-exclude-glass;

Themes which modify browser css must mimic what the default theme is doing as far as content is concerned. If content is not excluded, various anomalies can show up, most notably in windowed plugins which display improperly when placed on transparent window surfaces.

For reference:

If you’re a theme developer, please update your themes accordingly. This should only affect your theme if you use -moz-win-glass or -moz-win-borderless-glass. The AMO Editor team is aware of this situation and updates due to this bug are being given a high priority.

If you are a theme user and this problem affects you, we recommend that you check for updates (many themes have fixed versions available already), and let the theme developer know about this blog post. Disabling or uninstalling the theme should fix the faulty behavior, in case you need an immediate fix.

20 comments on “Firefox 6 breaking some themes when viewing plugin content”

  1. Karellen wrote on

    And some Mozilla devs wonder why many users want a long term stable “security updates only” branch…

  2. Glenn Howard wrote on

    I second the comment of Karellen to the effect that what’s called for is a long-term stable branch with security updates only. Cutting edge for the restless, reliablility for the rest. Certainly there’s nothing wrong with being in a hurry for the latest and greatest. But what’s wrong with wanting what already works to keep right on working?

  3. patrickjdempsey wrote on

    “This should only affect your theme if you use -moz-win-glass or -moz-win-borderless-glass.”

    Which means ALL themes as we are forced to use these appearances on the main-window in order for the window control buttons to appear on the custom titlebar. I’m guessing if we use -moz-appearance:none for the downloads panel, we won’t need to apply that code, but will we for the other two cases?


    @media all and (-moz-windows-compositor) {
    #clientBox {
    -moz-appearance: -moz-win-exclude-glass;


    @media all and (-moz-windows-compositor) {
    #appcontent {
    -moz-appearance: -moz-win-exclude-glass;

    My themes do not even have a browser-aero.css… do I need to create one for this to work? Also, would saying -moz-appearance:none and applying a solid background color work equally as well?

    1. Jorge Villalobos wrote on

      I guess this should only affect your theme if you override the default settings for any of those windows, but I’m no expert in this area. It shouldn’t be too hard to test with DOM Inspector to see if the -moz-win-exclude-glass style is being applied or not. You can try asking in the bug report for confirmation.

  4. Ken Barbalace wrote on


    Put the browser-aero.css rules for this issue in your browser.css file doing something like the following:

    —begin code—
    @media all and (-moz-windows-compositor) {
    #appcontent {
    -moz-appearance: -moz-win-exclude-glass;
    —end code—

    You should already have a “@media all and (-moz-windows-compositor)” section in your browser.css file to deal with the other aero issues we’ve had to contend with like the window control buttons at the top right of the browser window.

  5. P.A. wrote on

    There is currently a discussion on the group [1] on improving the method that’s used to track changes that affect add-on compatibility. This only a specific aspect of the more general discussion on how to better integrate rapid releases and the add-ons world, but improving this aspect will definitely have a positive effect for add-on developers.

    [1] “Ensuring that interface changes are documented for add-on compatibility”

  6. Energia wrote on

    Can’t you developers just leave the damn thing alone? I concur with Karellen and Glenn Howard: we all would simply prefer a consistent, secure, reliable platform.

  7. Jarrod wrote on

    So, any chance of fixing Firefox instead of waiting for Microsoft and everyone else to fix their themes? Seems only fair.

  8. Todd wrote on

    The devs might as well commit seppuku, because they’re literally making their browser worse all the time. Stop breaking things and start fixing things!!! If this browser gets any worse, I’m going to completely forget firefox ever existed and just use chrome and be done with these headaches….I care about stability and reliability, not new crappy so called cutting edge features that I (and many others) will never care about or use. All themes are practically broken as of 6.0.

  9. Dave wrote on

    Every new firefox version since 2.0 has had some annoyance like this and I’ve always been able to work around it with some new plugin or userchrome fix or whatever. I haven’t been able to find any workarounds here though. This could be the one that pushes me over the edge. Uninstalling firefox + installing chrome is starting to look like the best workaround at this point. More up-front cost, but with mozilla breaking stuff on an accelerated schedule now, switching could pay off rather quickly.

  10. Jess wrote on

    Firefox 6.01 is an even bigger mess as I can not go 3 page views without a crash. Stop rushing things and make them work before release.

    I am off to Chrome since I can not trust firefox anymore.

    1. Jorge Villalobos wrote on

      6.0.1 and the upcoming 6.0.2 are just security releases that modify the certificate DB in Firefox (see There are no code changes that would make it crash more or less than 6.0.

      1. Rej wrote on

        Not to rain on your parade, but updating to 6.0.2 killed my iGoogle page.

        It now displays differently & is missing some links that still show up in IE9 (“Advanced Search” & Language Tools for example).

        I’ve passe dthe last few hours trying to fix it and still can’t even after completly removing it, reverting to 5.x, 6.x & even the beta 7.x. I officially cannot revert to the way 6.0.1 was even when installing 5.0.

        If this was a security fix, it’s not part of the uninstall of FF so I have NO way to revert back to what I consider a working version of FF. Unless someone here knows how to undo what 6.0.2 did.

        Tried system restore without success, & NO addons/themes are installed.

        1. Rej wrote on

          Just realized that this isn’t the right forum/blog for this.. sorry 😉


        2. Jorge wrote on

          Rej: I don’t use iGoogle, but it sounds to me like the difference is the localized version of the page you are using. For example, if I visit Google in Costa Rica, I am redirected to, which may have different links than I’d look into that for possible solutions. But yes, it’s be best if your ask in

  11. Sammy wrote on

    You guys had a great product. Note past tense. Mozilla development has lost the plot. Forcing changes on your users and updating too often has completely ruined the Firefox browser. I’ve gone from enthusiastically supporting it to searching for a solution. If not for the addons I’d be long gone already.

  12. Larry wrote on

    I found a solution; most of the tech talk is over my head so I looked for a simple answer. I wetn into>tools>options>applications and selected Adobe Reader as the application to open pdf files vice Adobe Acrobat (in Firefox). No issues since a Reader window opens with the document not affected by the theme. Hope this helps someone.

  13. Liz wrote on

    for those of us who aren’t programmers, step by step, how do I fix this.

  14. Joan wrote on

    3.6 (now up to 3.6.22) is still working fine as a reliable browser. For the non-programmer rest of us, dump 4, 5, and 6 and return to the old-school fold. (7 is said to be way better, but it’s still in beta. I’m happy to wait.) Here’s where you can get the latest 3.6::

  15. Marty wrote on

    One would think that since so many people want to change the current Firefox browser to a much more appealing and stylish look like Total Glass that the people at Mozilla/Firefox would take a hint and make an option for such a style built into their browser.

    I have an add-on that does this now but it, like all the others I have tried is buggy. My icons are missing and I wish the tabs had some borders around them, stuff like that. But it occurs to me that Mozilla could do a much better job making it look right since it is their browser.