Okay, I've taken the plunge and upgraded to Firefox 3, at least on one machine—my Windows XP workstation at home.
I thought I'd give it a spin among promises of better performance. So far, most importantly, I haven't experienced anything not working.
I had to re-install the Google Toolbar which was simple and not a surprise.
I read through all of the features, release notes, etc. Nothing among the features is particularly exciting to me. It seems like there's some overlap between Firefox functionality and Google Toolbar functionality. I hope that doesn't cause any conflicts or confusion.
So far I haven't experienced any problems. It seems faster, but that's very subjective. I didn't think to check my memory usage before upgrading but, quickly firing up Task Manager, memory usage doesn't look recognizably higher than normal.
I'll see how things go. If all is well, I'll start to upgrade on other systems. I suppose the MacBook Pro would be next.