Seamonkey has an interesting history, in that it is both older and younger than Firefox. Older, because originally it was built from Mozilla Suite code (for those of you that don’t know, Mozilla Application Suite is the parent of Firefox, and was originally built from the code of Netscape Navigator which was open-sourced in 1998).
Seamonkey is also younger than Firefox in that Seamonkey’s first version, 1.0, was not released until 2006, 2 years after Firefox 1.0. Quite a few people are not even aware of the existence of Seamonkey or the Mozilla Suite, thinking that Firefox was the successor to Netscape Navigator, created deliberately to enact their vendetta against Microsoft for their monopolistic practices that killed Netscape. But glorious fantasies aside, Mozilla Application Suite was the real successor.
So you might wonder why Firefox might have been been created in the first place? Why not build on the existing functionality of their existing code. Well, one thing that they learned from Microsoft’s hostile takeover, is that the average user doesn’t like a lot of functionality, as that just confuses them.
Netscape Navigator was far more functional than Internet Explorer, and the Mozilla Suite failed to take off for the same reason. So the Mozilla developers started taking functionality out, deciding to focus on optimizing their new browser, Mozilla Firefox, for the one thing it was really intended to do: straight-up web browsing. Then they made prettier, simplified the interface, and voila! Now Internet Explorer had a web browser competitor to be reckoned with!