I had the same problem.
Windows 7 Pro new install (albeit a corporate install)
I did as Makondo said and started with a fresh profile (<Firefox install dir>\firefox.exe -profilemanager). I added TMP and the problem immediately started.
What I noticed though was that if I moved the mouse cursor outside of the pull down menu (I was using the Firefox tools pull down menu BTW) the pull down window would go into an outline with no content. When I moved the mouse back into the pull down menu it would start flashing wildly when I tried to scroll up/down the menu.
Then the light above my head went on and that behavior looked just like the pull downs on regular Windows windows.
So, I went into the Windows visual settings (Control Panel > System > Advanced system settings > Performance > Visual Effects Settings > Custom) and selected "Adjust for best performance" which turns everything off. Restarted Firefox and the problem is gone.
I am not sure which of the Visual Effects settings is the culprit. I did turn on the following Effects with no issues:
Show shadows under mouse pointer
Show shadows under windows
Smooth edges of screen fonts