Firefox's memory consumptio on Win XP Pro SP3 grows without bound. Have you checked for memory leaks recently?
I'm running the latest version of Firefox, with the usual plug-ins PLUS Roboform. I typically leave multiple browser windows open for days at a time, and have noticed that memory consumption increases even if I'm not opening new URLs until I kill Firefox and restart it. I just killed Firefox because after 2 days it was consuming over 1.25GB of RAM. This looks like a classic memory leak on the surface, so I wonder if anyone has tested the latest version for leaks.