If you want to reset your Firefox preferences, this article provides instructions on how to reset them through Safe Mode.

To reset preferences:

1. Close down Firefox completely: At the top of the Firefox window, click the File menu, and select the Exit menu item.Close down Firefox completely: On the menu bar, click the Firefox menu, and select the Quit Firefox menu item.Close down Firefox completely: At the top of the Firefox window, click the File menu, and select the Quit menu item.
2. In Windows, click Start, open the All Programs list, and navigate to the Mozilla Firefox folder. In the Mozilla Firefox folder, select Mozilla Firefox (Safe Mode).
   Firefox 3, Mac OS:

   Hold down the Option key, as you start Firefox.
   Firefox 2, Mac OS:

   Go to the Utilities directory (in the Applications folder) and open Terminal, then run:

   /Applications/Firefox.app/Contents/MacOS/firefox -safe-mode

   Go to your Terminal and run:

   /path/to/firefox/firefox -safe-mode

3. Firefox should start up with a Firefox Safe Mode dialog.
   
   Note: You can also start Firefox in Safe Mode by clicking Start, selecting Run (or use the Start Search box in Windows Vista) and then entering the following in the text field:
   

   firefox -safe-mode

4. To reset your user preferences, click to put a check mark by Reset all user preferences to Firefox defaults.
   
   
   
5. To apply your changes, click Make Changes and Restart.

Firefox will restart with your settings changed back to the defaults.