Firefox is excellent, but it's famously bloated. It'll be slow for a lot of people. Firefox 3 will be a big improvement, speed-wise.
I've found Firefox to be the most standards-compliant, with Opera being a very very close second. Firefox 3 and Opera 10 will be bastions of compliance. If a website looks weird in Firefox or Opera, that website was built by someone who had no clue what he was doing.
Safari has been a solid contender since version 2, but still has a couple small hurdles ahead (mostly javascript stuff).
IE isn't worth talking about until IE 8 (and even then, Microsoft could very well screw it up). It's the bane of every web developer's existence.
Opera does have the longest history of browser superiority (it's been around for a long time), but that's likely to change soon. Mozilla hired the people at Humanized (a usability consulting company owned by the son of the creator of the original Macintosh interface) to revamp the Firefox interface, which is a very big deal. The other browser companies will be tripping over each other to copy whatever they come up with.