Put them all in your "Login Items," with FireFox being the last (ideally), and check the checkbox beside all items except FireFox to be "hidden."
In reality, you can't control this. Different apps take different amounts of time to launch. In theory, you could launch Firefox, then while Firefox is launching, launch several small apps which would complete launching before Firefox did. So, even though you can launch applications "in order," there's no guarantee that they will finishing launching in that order.
I think your best bet is the Login Items option, unless you're open to more technical scripts and what-nots.