Frank Hecker wrote a quite long and very interesting article about this fundamental question. I say fundamental since answering the question is almost an attempt at forecasting Firefox's fate. Unfortunately, in my humble opinion, the article does not spend enough time on a key factor of Firefox being its great extensibility, just a few words to say it's extensible using simple Web standards and that IE was already extensible through HTCs/HTAs. From my perspective, Firefox's extensibility mechanism is a - if not the - key factor making Firefox a disruption. It is so simple and so powerful that it does allow (a) existing users to do what they already did in a cheapest/safest/cleanest way (b) application writers to do what they did in very different and/or more advanced ways (c) new users/app writers to do things they could not do in the past. HTCs/HTAs never reached that, never offered that flexibility - if you except the dangerous and extremely poorly documented "binary HTCs". You can do so much more with Firefox than with MSIE. Firefox extensibility is highly disruptive.

BTW, I already wrote on this blog that PhoenixFirefox is a disruptive innovation. That was more than two and a half years ago...