"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.

Firebender Ascension's ability will always go on the stack on top of the triggered ability that caused it to trigger. The copy it creates will be created on the stack on top of the original triggered ability and the copy will resolve before that ability.

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

If the ability is modal (that is, it has a bulleted list of choices), the copy will have the same mode(s). You can't choose new ones.

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.

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

The copy will have the same targets as the ability it's copying unless you choose new ones. You may change any number of the targets, including all of them or none of them. If, for one of the targets, you can't choose a new legal target, then it remains unchanged (even if the current target is illegal).

Triggered abilities use the words "when," "whenever," or "at." They're often written as "[Trigger condition], [effect]."