Uncle Iroh

Mana from firebending abilities isn't lost until you leave combat and go to your second main phase. It can be used at any time during combat, even after combat damage has been dealt.

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 {R}. Until end of combat, you don't lose this mana as steps end."

Any other mana you add during combat will still be lost as normal when moving between steps of combat.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.

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