From their project page: The Mozilla Firebird project is a redesign of Mozilla's browser component, written using the XUL user interface language and designed to be cross-platform.

I've used this for awhile and I can tell you it ROCKS. It's fast, and clean and an absolute joy to use. Those who like to use Mozilla but feels it's too bloated with mail, composer etc. would definately want to try it out. So give it a try it if you haven't already! The version I'm using is .06.