Once my system (with 1GB RAM) went slow, I checked my task manager, this is what i found.
My firefox was taking 314 MB of my memory.
I asked a friend, he said 'No, it takes 500MB+'.
This happens specially when you keep your system on without restarting or logging-off for days - memory leak. You can reclaim your memory by restarting firefox (closing it all and re-opening it).
Some told this issue will be fixed in firefox 3. Cool if that comes true. When is firefox 3 coming ?