When I launch FireFox for the first time, right when the FireFox dock icon starts to bounce, something else briefly opens on the Dock for a second or two, then the FireFox window will pop up.
I wonder if whatever the second process is doing is what causes FireFox to launch slower on OS X than Windows?
Doesn't FireFox show you a special "You just upgraded" page after you upgrade it? Maybe it's checking for a flag file on your HD to see if you're running it for the first time after an upgrade?
Below is a Dock shot of what I'm talking about. You should see FireFox starting to bounce towards the left of my Dock, and some weird document that briefly pops open immediately to the left of the Dock separator thing.