Players can respond to the triggered ability, but once it starts resolving and you decide whether to pay {4}{R}{R}, it's too late for anyone to respond.

The landfall ability triggers only if Akoum Firebird is in your graveyard at the moment the land enters the battlefield.

If, during its controller's declare attackers step, Akoum Firebird is tapped or is affected by a spell or ability that says it can't attack, then it doesn't attack. If there's a cost associated with having it attack, its controller isn't forced to pay that cost. If they don't, Akoum Firebird doesn't have to attack.

A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.

Whenever a land you control enters, each landfall ability of the permanents you control will trigger. You can put them   on the stack in any order. The last ability you put on the stack will be the first one to resolve (As a result, you can have those abilities resolve in the order of your choosing.).

A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.