Use top and hit "m"
That I like.
Shows Firefox using about 15%MEM
How often are you restarting FF? On Korora I found that it could not be left on for more than one day.
The system as a whole has only been running 1:4:3:00 - I restarted FF this morning after @dafyre made mention of it.
I mentioned the other day that you needed to restart it regularly ;)