Start Firefox up in safe mode. That disables all extensions. Stuff like what you're talking about, it's usually an extension.

Then, if right clicking while in safe mode works, you have to look at all your extensions one by one. Possible having to enable each extension, one by one, and Firefox requests a restart every time you enable an extension which sucks, but you gotta do it.

But, if right clicking while in safe mode works, just look at your extension list. The ones written by big companies like Adobe, Sun, Apple, etc. those are probably fine. Ones by Google and Yahoo though sometimes are problematic even though they're big names. Also, independently written extensions sometimes cause problems.

But, for all I know, you don't even have any extensions installed. In which case, safe mode won't make any difference.

You did reboot just to see if that works?

Basically what I'm saying is try Safe Mode. If that works and you can't figure it out from there, post your Extension List here.

What about your theme? Those are considered extensions too. If you changed the theme from the default, that could be causing the problem.