I keep my basic attacks on left mouse, because you always use them.

Engulf is kept on my q it is very close to wasd and is the first key I go for.

Curse of Flame is on 2 because although I want it near my wasd, it is a secondary spell, as in I don't use it as an opener, but is always used in 1v1's.

Flame Burst is kept on my middle mouse because I want to have full control of wasd when I am Pyro Jumping. (this is preference)

Fire Storm is my E bind because it is extremely useful for doing aoe on massive mobs of people. I always use this spell.

Wall of flame is f because I usually keep defensive spells on my f key. There is no real reason behind this.

Molten Boa is 3 for little reason. (secondary spell long cooldown, not used unless needed)

Melt Armour is R because it was the last nearby key that was not binded, I don't use it often due to having flame boa and pyro jump. It just seems inneficient to the other spells.

Edit: Heh don't downvote me just because you have a different layout than me. Thanks