Are all fire giants belly casters?
 

I seem to recall some of the named ones are

yes fire giants in Sol B are bellycasters

Am I the only one around here that doesn't know what the term "belly caster" actually means?

Stand under the belly of the dragon (or mob), i.e. melee range, to land spells.

Pretty sure you need to be standing on them to hit them. At least that's what I remember, could be wrong!

They'll resist all (or just most?) spells not cast within melee range.

These shoot fireballs out of their bellies. Beware.

This. If you aren't in melee range you aren't landing spells. That defines bellycaster. All the giants are bellycasters.

As far as the giants go: they(Verant devs) gave them this ability to make them more of a challenge.