This can happen if one of the files that stores details on installed add-ons is corrupt. Try the section on corrupt extension files in this link - https://support.mozilla.com/kb/Unable+to+install+add-ons

Though that link is about how to fix a different problem, the method for removing corrupt extension files may be able to fix your problem.

Thanks, unfortunately that didn't have any effect on the problem. :(

Just to be sure, you tried this?

Delete the files extensions.* (extensions.rdf, extensions.cache, extensions.ini, extensions.sqlite) and compatibility.ini in the Firefox profile folder to reset the extensions registry.
New files will be created when required.

See "Corrupt extension files": http://kb.mozillazine.org/Unable_to_install_themes_or_extensions

If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the "Find Updates" button (Firefox 4: click the Tools button at the left side of the Search Bar) to do a compatibility check.

You can also try: "Reset all user preferences to Firefox defaults" on the Safe mode start window.

Cor-el, that's what the guy above suggested, no luck. What's weird is that the extensions show as being installed and enabled in the addons menu, they just aren't working. At first I thought it was just a compatibility issue but if that were the case it'd say that in the addons menu, it doesn't. Deleting what you guys have suggested doesn't seem to do anything, it's really a head scratcher.

Can you make an individual extension work if you disable all others?

No, well, as far as I can tell all the ones related to Javascript updates are working but that's it.