I was wondering why my Firefox 2.0.0.6 was running slow (the past few versions have been a little slow too) and I figured it must be an add-on.  Turns out I was right.  After I removed AddBlock+ my surfing has been much faster.
I think having to check every image on a page to see if it should be blocked was just slowing everything down.
I get a few more adds, but a much faster experience.
I still use Flashblock to make sure I don’t get the annoying moving picture/sound ads, and that is good enough for me.