It makes sense I guess. The people who make firefox should be able to make a faster binary than a distro.
The mozilla firefox build is i686 rather than i386, and probably uses some other methods for extra speed that debian doesn't. Debian stuff is usually made for stability and compatibility first, rather than for speed.
However, so far on my machine the mozilla build has been running for a day or so. So it works nicely at least on my machine.