Does stifle stop a landfall trigger from resolving if it targets the trigger on the stack?

Stifle (and similar effects like Disallow or Trickbind) counters any activated or triggered ability. A landfall ability is a triggered ability — it triggers whenever a land enters the battlefield under its controller's control. Once that trigger is placed on the stack, it becomes a stack object that can be targeted and countered just like a spell.

When Stifle resolves and its target trigger is still on the stack, that triggered ability is countered (CR 509.2). A countered triggered ability is removed from the stack and never resolves, meaning its effect is completely prevented. This is governed by the general rule that countered spells and abilities are removed from the stack without effect (CR 700.3a).

It is worth noting that Stifle does not prevent future landfall triggers — it only counters the single instance of the trigger that it targeted. If another land enters the battlefield later, the landfall ability will trigger again and a new stack object will be created.

Concrete Example: You control a Lotus Cobra. Your opponent plays a Swamp, which causes Lotus Cobra's landfall ability to trigger and go on the stack. Your opponent responds with Stifle targeting that trigger. When Stifle resolves, the landfall trigger is countered — you do not add any mana to your mana pool from Lotus Cobra this time.

The relevant rules are CR 509.2 (countering triggered abilities) and CR 700.3a (countered objects are removed from the stack without effect).