So, if Firefox crashes on your Mac OS X system and upon restart starts outputting audio to... oh, let's say, a Plantronics USB headset instead of to your normal speakers, just unplug your headset and restart Firefox. The application will tell you that it has created new settings and will ask you if you want to use the new or original settings. Pick new, wait the 15 seconds for Firefox to do who-knows-what, plug in your headset again, and all will be well again. What you shouldn't do is try to find an audio output setting in Firefox or turn the volume on your mixer up and down repeatedly.
Note to Adam Nash: I am not bitching. I just couldn't find this information on Google and therefore felt compelled to post a solution so that others won't have to waste time should they have the same problem.