Try starting Firefox from the command line to see info on what's going on during startup. In particular, you might want to use firefox -safe-mode to see if you get a fast startup that way. If so, then it's probably an add-on causing the delay.

Incidentally, since someone else has suggested that a slow startup is somehow normal for Firefox, let me point out that Firefox is known to have a fast startup on 11.10. At least, according to this Tom's Hardware testing, it beat Chrome and Opera when loading one tab. Personally, I find the startup very swift on Precise beta2.