Usually that means that firefox memory usage has gone very high, probably due to a memory leak from one of your addons/plugins and you should restart firefox fairly frequently to get around this. You could try using something like Memory Restart to inform you when memory usage has gone too high and restart firefox. I’ve turned off the automatic restart option, and just use it to inform me of the memory usage.
Got it solved for tonight but need to update this box soon. Thanks! God Bless!