I experienced the same problem a couple of days ago with my MBP and the latest version of Firefox. No theme changers were installed. Ultimately, it even stopped letting me clcik in the address bar to type in a new URL.
A restart did fix the problem. But there does seem to be an issue where if Firefox has been left open for too long or if you have a certain number of tabs open, it will stop responding to mouse clicks for some (or even all) functions.