I used Opera primarily for a few years, then switched to Firefox, and nothing will make me switch browsers except for a GOOD vimperator equivalent/port.

Used Opera waaaay back, then Firefox since it's earliest stages, but I feel it's getting worse with every update, so I'm back to Opera, and I'm really impressed where the guys took it, I'm loving it... except for the occasional compatibility issues, but that's more a problem of the websites, not the browser.

Firefox, accept no other

I used Chrome for a long time, but then it got really buggy. Switched to Firefox, same problem, used then Opera, great browser, but I had some compatibility issues with some websites.
Now I am back with Firefox 8 beta and it works great, no crashes, no weird behavior , Opera is still my #1 but Firefox has more addons