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:
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.
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.