I've been fighting a sluggish Firefox all day today. I've had to use the dread Task Manager a half dozen times to close down my processes and start again. Just now, just at the end of the day, Firefox manages to produce a pop-up alert to warn me.
There is a busy script; chrome://google-toolbar/content.gtb.js:218.
I'm pretty sure that's the bookmarks script (which I've started to use more and more) now that I've lost the bookmark sync with my upgrade from Firefox 2 to 3.