Firefox isn't shutting down properly, I have to kill it in System Monitor. It has happened twice this evening. I see that others have had this problem but those seem to be much older distros than mine.
I am running 12.04 and Firefox 27.0 on a 64 bit system. There are no add-ons and I haven't established a profile, it is exactly as it comes out of the box.
Earlier sufferers of this problem (4 years ago!) identified a bug in Ghostery but as I said: I have No Add-Ons.
If it helps, I had a similar problem with Compiz several days ago, that problem has not, so far, re-surfaced.