[ubuntu] Firefox tab bar color changing

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http://ubuntuforums.org – I'm using the Ambiance theme on Ubuntu 12.04, and this is what I normally see when I open Firefox: However, sometimes it changes to this: which is really annoying. I find it aesthetically unpleasing and as you can see the title text of background tabs remains light-colored, thus becoming unreadable on this light background. It used to only happen when I added or modified a bookmark, so I just didn't do so, but ever since I upgraded to Firefox 20.0 earlier today it's been doing it whenever I download a file and sometimes when I open a new tab. (Also, as you can see I don't use the bookmar (Hardware)