when you install it, there's a BIG chance it pops up there automatically
this means, though, that you'll have to restart your Gnome Panel "for the changes to take effect", as they say. either restart X (ok, don't do that - too big thing to do for a small task
heh), or just gnome panel....open up a terminal window, and give command:
then watch as your gnome panel goes away and comes back. check if you got your firefox there
if not, then just click your menu open, click Applications menu open, click Internet menu open, and on that menu click with your 2nd mouse button, select Entire menu -> Add new item to this menu and then fill in what you're asked and you're fine
hopefully this helped.