Boiling Rock Rioter

Firebending abilities aren't mana abilities. They use the stack and can be responded to.

"Firebending N" is a keyword that represents the ability "Whenever this creature attacks, add N red mana . Until end of combat, you don't lose this mana as steps end."

Multiple instances of firebending on the same creature trigger separately, each granting you the appropriate amount of mana.