Opera is cool but as in constant development it has a lot of bugs! they should focus on security. Chrome is fast and very quick when I need something, Firefox has good extensions and is very compatible with many applications I use, so my final answer is Safari, haha I'm kidding, I use them all!... just that in different cases...
Unfortunately, Firefox 4.0 doesn't support SEOquake as it's still in beta. Thus, no SEO friendly at all.
Ive gotten so used to firefox that i doubt ill ever change, one downfall is that it down crash now and again which is annoying.