hello Matt58, maybe the file that holds the customization of the user interface is corrupted or one of the addons is interfering.
in order to reset the UI, go to firefox > help > troubleshooting information, click on profile folder/show folder and close all firefox windows afterwards. a windows explorer window should open up - in there delete the file named localstore.rdf - it will be regenerated again the next time you launch the browser.
otherwise if the former doesn't work, you may have to go through your extensions and disable them one by one until you find out which one is causing this...
Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.
(If you're not using it, switch to the Default theme.)
On Windows you can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
On Mac you can open Firefox 4.0+ in Safe Mode by holding the option key while starting Firefox.
On Linux you can open Firefox 4.0+ in Safe Mode by quitting Firefox and then going to your Terminal and running: firefox -safe-mode (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)
Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.
Once you get the pop-up, just select "'Start in Safe Mode"