I don't care opensource/evil M$/Opera the fastest browser in the universe, I just use whatever I feel comfortable.
For now, I use IE 6.0 and Opera 9.0 everyday. I used to fan FF a lot but this bloatware drives me nuts and those extensions eat memory like hogs. My favorite is absolutely Opera 9.0 so far. It's fast, secured, with many neat features (like page zoom).
If FF can start as fast as Opera/IE and cures its damn memory leak (com'on, hire some good programmer please! or lure someone from Microsoft), I would use it again, along with Opera and IE.