A firegasm is a orgasm experienced by certain people when they see a fire. Mainly experienced by pyromaniacs.
Pyromaniac 1: Look at that massive fire
Pyromaniac 2: Dude, i just had a firegasm
Pyromaniac 1: Sweet
Emotional climax upon viewing of dangerously impressive spectacle involving fire.
I know! I am having the most intense firegasm right now!