I switched from Opera to Firefox, despite Firefox being noticeably slower, because of extensions. I love extensions. In fact, right now I'm unable to imagine a web without extensions. It would be like using a OS without the ability to write scripts or code to interact with it. I realize this probably doesn't apply to all Firefox-users, but I seriously suspect it applies to almost all early adopters.
Why would I switch to to Chrome with extensions lacking, when it offers me nothing more than Opera? In fact Opera offers more than Chrome, is just as fast (based on actual machine-to-human metrics, not artificial millisecond wanking) but that didn't stop me from going to another browser.
Speed is only one factor in the equation, and on modern machines it is highly irrelevant. If speed was that important to everyone, why didn't Opera turn out to be the top dog years ago?