Ozai, the Phoenix King

As long as Ozai, the Phoenix King is on the battlefield, you'll keep unspent mana as steps and phases end (although it will become red). This means you can add mana and spend it during a future step, phase, or turn. Once Ozai leaves the battlefield, you have until the end of the current step or phase to use the mana before it disappears.

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 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."

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

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

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