What process is taking up that 20-25%? Cant help you if you dont tell us that kind of info.
Have you tried to go thru our guide?
http://www.techist.com/forums/f51/spyware-removal-guide-osiris-updated-11-13-2008-a-165828/
Get the Firefox Ultimate Optimizer.
Firefox Ultimate Optimizer - Download
That will reduce the CPU usage of Firefox.
Are you using the latest version of Firefox? 3.0.5 is the latest stable version. Check that. If not update to the latest version. See if it helps. There was just a major batch of updates done to Firefox this week. Updated just the other day. So it couldn't hurt to look.