Guild Wars 2

It clearly says “each fire spell you cast grants you might for 10sec” and not “each fire spell you cast in fire attunement grants you might for 10sec”

I tried using flame ax / FGS while in non fire attunes and I did not get a might stack.

Did you tried to use flame axe in fire attunement??

I did. I got the might stacks when I swapped to fire attunement.