I have used FF since 1, Opera since 9 (tried out before, but didn’t like it), safari since 3 and IE since.. well windows 3.1 was my first OS. (I used Netscape some time, haha)
And although I find Opera faster and lighter than any other, FF3 does offer better features for me, and they do concentrate on the little details that makes my geek part say “niiiice”.
The ability to use spell check by simply add a dictionary, a better “find” tool, more customizable panel (having the menu, buttons, address bar in the same bar).
Little things, like asking to remember passwords with a top bar, while you keep browsing instead of stopping the transfer and asking you with a pop up window.
And the little spark at open source seduces me.
So I still use Opera a lot, I still have it as default browser, but I hope that FF improves some more and I wont even think about it. I also use FF in any place with the portable apps suite.
Cheers!