Do Not--DO NOT--Upgrade to Firefox 1.5 (or is it 5.1? They don't seem to know). It is horrendous. None of my toolbars work anymore, and I can't type in a word processor and copy the result to Blogger.
By the same token Opera is a nice, fast interface, but it doesn't work well with Blogger at all (the toolbar contains exactly zero tools).
I don't care much at all for Explorer, but I'm back there until Mozilla gets its act together.