I have used FF since 1.0. I typically have 3-6 open instances with 50-70 tabs. The only add-ons I use are Adblock and Noscript.
I tried FF3 for a couple days and ripped it out (its not trivial to downgrade) I run it on XP Pro SP2.
1. Despite all the hoopla FF3 used the same amount of memory as FF2, not that this really matters.
2. FF3 constantly consumes 20%-40% of the CPU when nothing is going on. FF2 displaying the exact same pages consumes 5%-15%.
3. The awesome bar sucks. My biggest problem is that it displays what you have been browsing to anyone looking over your shoulder. Lets imagine you have have been visiting a jobs site. You boss walks up and asks you to pull up something. You create a new tab and start typing "www". You get that far and FF3 displays a huge banner with hotjobs or whatever. What a stupid idea. Its even stupider that you cannot revert to the FF2 behavior. The suggested fixes (oldbar addon and various tweaks) don't really restore FF2 functionality.
4. FF3 sometimes takes minutes to start.
I'll be running FF2 until Mozilla fixes these problems. If they don't I will abandon FF.