Recently I upgraded to Firefox 18.104.22.168 on Mac OS X. Ever since then, Firefox has been crashing extremely frequently. Most often, it's when I'm closing a tab---usually GMail.
I just reproduced the problem frighteningly easily: after a crash I restarted Firefox. I then went to GMail. After the page loaded, I closed the single tab containing GMail. Crash. Ouch.
What is keeping me from switching to Safari or Opera? At this point, familiarity and Google's bookmark synchronizer. The synchronizer has its own problems, though.