Ah, okay.  I gave an explanation when asked in the skype chat, but I hadn't realized that it was a request in the thread proper.  Here's what I said:

Me) Firebrand Trigger is a molten enhancement that offers several baseline advantages, but its greatest strength is the way it adapts to a person's own talents.
Kafka) Is one of those baseline advantages resistance to fire?
Me) I'd say immunity to the ordinary fire element.
Something like hellfire might still be a problem, or some sort of holy or dark fire, but normal fire doesn't compare.
When active, Firebrand Trigger gives the user's body a fiery aspect.  It alters skin to a fiery, stony appearance.  For physical fighters, it might change armor into sculpted volcanic stone and fuse it with the skin, creating a lava golem-esque appearance, and perhaps enhance musculature and/or size.
Magicians might receive an effect on their clothing that makes it look is if it were burning, or seeded with embers, which increases magic rather than physical protection.
Weapons could also be enhanced.  A two-handed longsword might become a sharp-edged slab of magmatic stone, always burning.  A magic staff might develop a fire gem at the tip that glows like a furnace, adding fire to spells.
Basically, Firebrand Trigger offers a lot of player freedom to tailor it to your own character.