That's not how I remember it. Firefox (Phoenix / Firebird) initially got its userbase from the Mozilla userbase which was this capable browser with many featur

That's not how I remember it. Firefox (Phoenix / Firebird) initially got its userbase from the Mozilla userbase which was this capable browser with many features and flexibility (extensions), but it was bloated, slow, with a kinda outdated (non-native) UI. Firefox's differing feature was its lightness and (native-like, per-platform) UI freshness, not configurability. FF success against IE was mainly caused by Microsoft completely dropping the ball by simply not developing IE for a couple of years. I think one of the elementary features which has driven mass FF adoption was tabbing support, which IE6 didn't have.