Jolly -> + Speed - Sp.Atk
Naive -> +Speed -Sp.Def
I don't know why you are going to put a Jolly nature in a Fire Blast M-Salamence.
So Naive is the best nature, gives you Speed and don't decrease Sp.Atk!
Also Dragon and Flying types Pokémon are OHKO by Ice-types each are very common (Ice Beam??) and Fire Blast can cover from them.
Also like Mr Gamaken said, Flamethower is better than Fire Blast because of the accuary, it will be very bad if your Fire Blast misses.. but if your Salamence is a 4 special moves, you should put a Timid or a Modest nature which is rare to see, and if you want to give a mix-salamence (both physical and special moves in a set), Naive, Lonely or a nature that decrease Defense or Sp.Def is great, but have a nature that decreases Sp.Def because M-Salamence will have the Defense stat very boosted (80 ->120).
The only physical fire-type that Salamence can learn is Fire Fang (65 BP).
Remember you can use Dragon Dance with Fire Fang! M-Salamence has more Attack than Special Attack by 5 BST (145 Attack / 140 Sp.Atk).
Hope this helps.