ProwessProwess is a triggered ability. “Prowess” means “Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.”
If a creature has multiple instances of prowess, each triggers separately. (CR 702.108) (Whenever you
castTo cast a spell is to take it from the zone it’s in (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. A player may cast a spell if they have priority. See rule 601, “Casting Spells.”
To cast a card is to cast it as a spell. (CR 701.5) a noncreature spell, this creature gets +1/+1 until end of turn.)
FirebendingFirebending is a triggered ability. “Firebending N” means “Whenever this creature attacks, add N . Until end of combat, you don’t lose this mana as steps and phases end.”
An ability that triggers whenever a player firebends triggers whenever a firebending ability they control resolves. (CR 702.189) X, where X is this creature's power. (Whenever this creature attacks, add X
. This mana lasts until end of combat.)