I've been using firefox for 12 months and just tried I.E. for no particular reason and was really taken aback to find that IE is MUCH faster than Firefox.
I've never noticed any difference between the two, except in one aspect: if you enter a non-existent address, Firefox just hangs for ages and ages (well mine does!).
stevie: it shouldn't really slow down that much due to those files. perhaps a little though if it's storing 12 months' worth. goto tools->clear private data in firefox to erase them from time to time.
catso: ive not found that, except if you enter an address without the http or www parts. if you just enter something and it doesn't contain either of the about two it goes to google and does a search for what you typed in, then sends you to the first link google finds. often of course google is correct.
the thing that annoys me is that not all servers are setup to go straight to example.com instead of www.example.com, so if you miss out the www bit it takes a short while for the server to process things.