Seriously that guy will mess you up.

Actually,Martians just fear fire.It's kind of like Kryptonite to them.They don't even have to touch it.As long as it's around them,they go down.

Honestly it depends on what storyline you're reading. At times it was an psychological fear created by the Guardians, other times it's a phobia he developed after witnessing the death of his race and in some versions, fire itself is so chaotic that it messes with his telepathy or something.

Manhunter is one of my favorites, especially beacuse he can shape-shift

Well,M'Gann has the fear too,you know,Miss Martian?I've only seen the 'me see see fire,me scared' kind of Martian.