I noticed this at home when I got the FF updates about a week ago. I'm using XP SP3. Sometimes it snaps to attention and other times it thrashes the HD and doesn't load the pages right away. FF worked very well before this week. I'm clean as to malware and other crap.
It could be the new release. I don't have anything to compare it to as my machine's W7RC and FF3 is a clean install on a fresh HD. Plus, I installed only the latest FF3 release - I believe it's only a few days old.
I've got other machines in the house running W7RC, but they don't have FF3 installed. Maybe I'll try the previous FF3 version and see if it's working any better.
I had FF3 on the 7000 build of W7 and it worked without any problems for almost 6 months. I've also noticed that the FF3 Adobe Reader plugin crashes with FF3 and not IE8 as well.