It happens, though usually it's a once every so many blue moons kind of thing. Griff's advice is pretty good, and I'd just add to that now is as good a time as any to audit your extensions and plugins, decide whether or not you really need all of them. If not, get rid of them.
And as a temporary workaround, you can always use task manager to forcefully kill the firefox process. Just don't be too quick on the trigger, since as Grif mentions, there are legitimate "tidying up" tasks programs tend to do when shut down. The exact amount of time you should wait depends on the particulars of your system, but I'd say 5 minutes is a good general cutoff point. If, after 5 minutes, it's still going... That's USUALLY a long enough good-faith length of time to wait.