Firefox – How to disable Aero transparency in Firefox 4

There seem to be two main (known) culprits here.

1. Desktop Gadgets (sidebar.exe). If you can live without them, try disabling them:
   Right click desktop > select "View" > uncheck "Show desktop gadgets". This didn't work for me.
2. Microsoft Intellipoint v8.1+. This was the solution that worked for me. It seems when they added support for the Microsoft Touch Mouse, the updated Intellipoint software introduced this bug. To fix it:
   Go to "Add or remove programs" and uninstall Microsoft Intellipoint. Instant fix, no reboot required.

If neither of those solutions work for you, I would try closing all open applications except for something innocuous, like notepad. Then open up your task manager and kill off suspicious processes one at a time until the problem is resolved. Obviously, make sure you don't kill a process that will destabilize your system. This could be a very tedious task.

This last post on the page in this thread helped me find the answer.

I had this same problem. No need to uninstall any updates, you just need to reboot your computer again, a second time (after the automatic reboot following the Windows Update), and you get your usual theme back.

See: social.answers.microsoft.com