Does landfall trigger when a land enters under your control through an opponent's effect?

Landfall is an ability that triggers whenever a land enters the battlefield under your control. The source of the effect — whether it is your own spell, an opponent's spell, or any other game action — is irrelevant to whether the trigger fires. What matters is only that a land entered the battlefield under your control (CR 702.91a).

Triggered abilities watch for a specific event to occur and automatically trigger when that event happens, as described in CR 603.2. The event landfall watches for is a land entering under your control. There is no additional requirement that you were the one who caused it.

It is also worth noting that the landfall trigger checks the battlefield state at the moment the land enters. As long as you control the land when it enters, the trigger will occur, even if you lose control of it immediately afterward.

Concrete example: Your opponent controls a card like Tempt with Discovery and puts a land onto the battlefield under your control as part of that effect. If you control a Lotus Cobra, its landfall ability will trigger, giving you one mana of any color — even though your opponent's card caused the land to enter.

This is consistent with how all triggered abilities work: they do not care about intent or who caused the triggering event, only that the event itself occurred (CR 603.2, 702.91a).