I have been having problems since release 17 (at least) with Firefox sucking up CPU. I have 4GB ram and often have multiple programs open, including Firefox with between 5 and 15 tabs / windows. Decreasing the number of open tabs / windows will decrease CPU usage by 2% for each tab / window closed until it hits 30%, then no more improvement. Oddly enough it drops to a constant 25% when Firefox is NOT in safe mode and all my plugins are enabled. But not below.
Help. I love firefox, but it is becoming more and more problematic.