I guess you could say its a strip down version of mozilla like without mail client...ect
And I think that thunderbird is for MacOS's

According to the website ...

Mozilla 1.5 / 1.6b - latest browser (windows/mac/nix)
Firebird 0.7 - next generation browser (windows/mac/nix)
Thunderbird 0.4 - e-mail and news browser
Camilo 0.7 - browser specifically for Mac OS X

Why is there a separate browser specifically for the mac when Mozilla and Firebird are both already compatible with the Mac? Are Mozilla and Firebird two different browsers that are branching development? Will Firebird technology eventually be used in Mozilla? I'd like to put something on my Windows machine and something on my Mac. Should I install Mozilla or Firebird in Windows? Should I install Mozilla, Firebird or Camino on my Mac? They are all mac-compatible!

It's hard to quantify, but at the most basic level Mozilla is stable (but evolving), whereas Firebird is sill considered beta, prerelease and experimental.

Remember, Firebird is a stripped-down browser-only package that is currently at v0.7. There's a warning on the Mozilla.org site not to use it for mission-critical applications. Some of its added features may well be crossed back to Moz when appropriate. That having been said, somebody has to run Firebird and/or Thunderbird for test, and a number of people do.

The stated objective is to carry on the Mozilla, Firebird, and Thunderbird development in parallel until Firebird and Thunderbird are at 1.0 release, at which time Mozilla will be dropped. That means that, for now, I will stick with Mozilla; it's likely to go through a few more versions. This is especially true since I use all the features -- browser, mail, and Composer for my websites (hey, they're nothing fancy).