Did a little research: IE may be better at sharing resources, or "backing down" at competing for microprocessor time. If you are sure your computer is virus free, try this:
Hit Ctrl-Alt-Delete to open your task manager.
Click on the processes tab.
Right-click on the Firefox browser.
Click on the "priority" tab.
Set the priority to lower.
When you have multiple applications competing for processor time, this will make Firefox yield time to more applications.
I'm virus free. So is my computer. I'm runing McAffee Security Center. Seems to work.
Something else I've noticed is when I am at a webpage that uses JAVA extensively (like NOAA.gov for weather radars), that tends to jack up the amount of memory that FireFox is taking up.
The only true way I've found to get it to back off is to close it and reopen the entire application.
And it isn't NEARLY as bad now as it used to be.
The resources are settings in Firefox. Think multiple tabs, each with a history, cache, etc., and Firefox keeping it all in memory for quick access by you. You can tell Firefox not to keep so much around.