This sounds like it may be caused by an extension.
Try the Firefox Safe Mode to see how it works there. The Safe Mode is 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+ Safe Mode by holding the Shift key when you use the Firefox desktop or Start menu shortcut.
Or use the Help menu item and click on the Restart with Add-ons Disabled... menu item while Firefox is running.
Don't select anything right now, just use "Continue in Safe Mode."To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shift key) to open it again.