I've found chrome to be just as bad a memory hog on non-topgrade systems as firefox, and often times is even slower during address typing/predicting.
As for firefox, the number one recommendation I can make is limit the number of tabs you open, and close the whole thing down frequently. Bastard bloats up as if it has a memory leak real bad when left open. My firefox experience got a whole lot better when I started just Xing out instead of minimizing it. Bear in mind that despite "vanishing" it still takes a minute or so to unload itself from memory. Sometimes, if I've had it open a long time, I just say "screw it" and kill it in task manager instead of closing it gracefully. I've yet to suffer an ill effect because of doing so.