Yeah, it has a lot of extensions which a lot of people value highly. Personally, I don't miss the extensions in either Safari or SeaMonkey. Firefox to me just feels "heavy" and non-agile, and has a Windows look to it.
I agree with you completely - these are the same reasons why I don't use it as my main browser.
I really like the extensions that are available for Firefox, but there's little consistency from extension to extension so the interaction changes slightly with each one - which results in a clunky, incoherent feel. Mostly, I have small apps that can do the job just as well and aren't there when I don't need them.