Well all of mine which are IE7, Msn, and firefox.(main problem is with firefox though)Well, firefox IS slow unless you tweak it.
If you have tweaked it, it still has memory leaks that will cause it to bog down your system over X amount of time.
That being said... if you are closing it every so often and it's still attacking your CPU usage like you say, there could well be another issue.
Do you have any viruses?
And what other browsers are you experiencing issues with?