Phoenix dropped XPFE in favor of lighter XUL that relied more directly on OS widgets. Here's a message talking about it:

http://sourceforge.net/p/xul/mailman/message/1494849/

I think Firefox mostly started out from an urge to make a "Browser product", rather than whatever the hell the suite had become. It shows in the release notes:

http://website-archive.mozilla.org/www.mozilla.org/firefox_r...

A bunch of stuff about convenience and fast and fixing problems and usability is in there.