Create a new profile as a test to check if your current profile is causing the problems.
See:

• Basic Troubleshooting: Make a new profile

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins"

If that new profile works then you can transfer some files from the old profile to that new profile (be careful not to copy corrupted files)

See:

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Though this may be potentially the solution for this problem, I can't fathom why there is even a situation in firefox that would 1) change a profile that would render the browser un-usable and 2) that Mozilla would stand by and just let us try to figure out how to fix an apparent flaw in this version. From reading through the threads, LOTS of users are experiencing this same issue. I didn't have to worry about actively setting up a PROFILE when I started using firefox and now I not only have to create a new profile but investigate how to move stuff from one profile to another. I absolutely LOVED the firefox browser, but have been using the stupid IE because there is just so much to figure out to fix this Mozilla problem. Disappointed.

Here's another possible solution: Firefox may have gone into Safe Mode for reasons that I don't understand. But if you open your Mozilla program folder and click on Firefox Safe Mode- you would then normally get a Safe Mode Dialog Box that allows you to reset stuff. When I clicked on this, I did NOT get the dialog box, but magically it launched a browser with all the buttons working: back and forward, refresh, menu were all restored to working order. Don't know if this is the real solution, but it gets me going. Worth a try for those who have experienced this problem.

In Firefox 4 you enter Safe mode if you hold down the Shift key and double click the Firefox desktop icon to start Firefox.