History of the Mozilla Project — Mozilla
it was a fork that took over (in popularity) by stripping out excess features (namely the integrated mail client) to load web pages as fast as possible. still under the umbrella of the mozilla corporation, i don't really know much about the corporate history *shrugs*.
they also got a lot of press about being immune to scripting/hacks... but after getting popular (and having enough user to merit specific attacks), that proved to be untrue and they've slowly bloated the thing up too. chrome has faster load times for me, and unlike firefox, doesn't sit open in memory after i close it (firefox does that so subsequent browser openings happen faster, or at least, it used to).