Its actually called: "mozilla firebird" for a reason.
There is the full mozilla browser, it has lots of things in it.
firebird is an offshot of it, its the same code only stripped down to the bones. and cleaned up and improved...
they use the same rendering engine... 'etc.
yes, with time they will grow apart. but right now mozilla firebird/firefox is about the same as mozilla only faster.
Best,
Eugene.