Do a clean reinstall and download a fresh Firefox copy and save the file to the desktop.

• Firefox 5.0.x: http://www.mozilla.com/en-US/firefox/all.html

• Uninstall your current Firefox version.
• Do not remove personal data when you uninstall the current version.

Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer.

• It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.

Your bookmarks and other profile data are stored elsewhere in the Firefox Profile Folder and won't be affected by a reinstall, but make sure that you do not select to remove personal data if you uninstall Firefox.

Didn't work. I've even tried making a new profile.

Actually, I'm thinking there is something wrong with my mouse. I'm getting incredibly weird scrolling going on, where the mouse scrolls up when I move the mouse or press any button. Ironically, the Firefox window is the only window where this isn't happening (luckily as well, pages would be unusable).

So I'm thinking the in-ability to use drop-down menus is somehow related to the mouse having issues.

Alright, I've plugged in a new mouse, and this doesn't happen any more. Issue resolved.