Try the Firefox SafeMode to see how it works there.
A troubleshooting mode, which disables most Add-ons.
(If you're not using it, switch to the Default Theme.)

• You can open the Firefox 4.0+ SafeMode by holding the Shft key when you use the Firefox desktop or Start menu shortcut.
• Or use the Help menu item, click on Restart with Add-ons Disabled... while Firefox is running.
  

Don't select anything right now, just use "Continue in SafeMode."

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shft key) to open it again.

If it is good in the Firefox SafeMode, your problem is probably caused by an extension, and you need to figure out which one.
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

When you figure out what is causing that, please let us know. It might help other user's who have that problem.

I got it working again. I (think) I used safe mode to do it. I do remember that the add-on options was missing from the drop-down menu. In any case, I got into a mode that let me delete all add-ons. When I got Firefox back, the only one left was from McAfee. I disabled it and things are back to normal now. So I think it was probably related to the known McAfee site advisor problem. However, the add-on is disabled, but Site Advisor is still working. Hmmmm.