You'd expect Firefox to be faster than Mozilla. Other than that, it's going to depend on a great many other things: what other services you're running (yep, there will be a lot of them running from a cold boot), your X set up and on and on.

You don't say how you're starting Mozilla, but FWIW, with an uptime of 4 days, starting from the taskbar until the home page loads, running the stock Debian Moz 1.5, takes approx 11 seconds.

System

Athlon 2.0 XP
1/2 gig pc2700 RAM
1/2 gig swap