Ive stopped using FF3 for the exact same reason (Mac BTW), Ive switched to Camino which based on Mozilla's Gecko engine, uses less resources. Im going to play with the 3.1 beta for a bit and see how that fares this week.
There are a few *fixes* for the memory leak by editing the about
:config but they didnt make much difference for me
How to fix the Firefox memory leak (Firefox hack) « // Internet Duct Tape
Flash can sometimes bog it down a bit as well, adding flashblock if you havent already might help a bit.