Looking for a name of a gigantic firebased mythological beast

If I understand this wiki article correctly, they’d be called hrímþursar, ‘rime-giants’.

Unless I’m very mistaken, “Haseraphim” is simply Hebrew for “The Seraphim.”

Other suggestions:

Prometheans.

Svarogs, from the Slavic fire god.

Azer (another roleplayer, here!).