Hmm, you just made me wonder if it's an accessibility thing. You didn't do something to change any of your computer's accessibility/vision impaired usability options, by any chance?

The only other thing I can think of is that something happened to change your browser theme? Changing themes has changed my tab background before.

Last question (probably obvious, but it's always the last thing I want to try). Have you tried restarting firefox in safe mode? That will at least tell you if an add-on might be to blame.

Otherwise, I'm stumped. Good luck.