What version of Firefox are you using? There was a version a couple weeks ago that snuck passed updates-testing into stable that has a nasty memory leak. I ran into it with oomd killing it off, so I masked oomd. Haha, bad idea, the memory leak locked the whole setup, had to force power off (I gave it a few minutes, which is really too long). I think the FF bug got fixed but maybe not always? So as it turns out oomd is doing the right thing in this case, just that we're not getting a desktop notification about what happened.
-- Chris Murphy