[daniel02216]

A pessimistic answer would be that all the people who were working on Mozilla who liked to add bloat like email and IRC and Palm syncing to a web browser jumped over to Firefox. Or that the guys who wrote Firefox 1.0 have moved on. Or that that's what happens when you write your UI in XML, render it with a browser engine, and load half a million extensions and history databases from disk on startup.

A different answer might be that it does so much more that a slowdown is part and parcel of doing all that's necessary in a modern browser.