For most people, I would suggest trying to run Firefox in Safe Mode first.  You can use the command line:  firefox -safe-mode to get there (for Windows users).

On Linux, you can go to your Terminal and run (for Firefox): /path/to/firefox/firefox -safe-mode

On Mac OS X, go to Utilities (in the Applications folder) and open Terminal, then run (for Firefox): /Applications/Firefox.app/Contents/MacOS/firefox -safe-mode

From there you can disable add-ons, reset toolbars and controls, reset bookmarks to Firefox defaults, reset all user preferences to Firefox defaults and restore the default search engines.

What if Safe Mode Does Not Help?

If that does not do the trick for you, you might want to try opening up a clean profile to see if you are still getting the problem.  Check out this previous post to fine out how to launch the profile manager to do so.

Key thing to remember here is if it is a problem with all of your Firefox profiles (your current one or a new one) then you might want to backup and do a clean install of Firefox.  If not, then you might be able to cure you problem by creating a new profile for yourself to use from now on.