If my opponent casts Murder on Feldon of the Third Path and I respond by activating Feldon's ability targetting the Mystic Snake in my graveyard, will the snake's ETB trigger be able to counter my opponent's Murder, or will it resolve too late?
When Feldon's activated ability resolves and brings Mystic Snake to the battlefield, that doesn't make Murder resolve right after that, but rather, first, Mystic Snake's triggered ability (which triggered when Mystic Snake entered the battlefield) goes to the stack above Murder, so that it can target Murder this way (C.R. 117.3b, 117.5, 405.2, 603.3, 603.3d, 601.2c, 603.2).
Remember also that the stack doesn't resolve all at once, but rather spells and abilities in it resolve one at a time, and only the topmost spell or ability on the stack can resolve this way (C.R. 117.4).
When Feldon's activated ability resolves and brings Mystic Snake to the battlefield, that doesn't make Murder resolve right after that, but rather, first, Mystic Snake's triggered ability (which triggered when Mystic Snake entered the battlefield) goes to the stack above Murder, so that it can target Murder this way (C.R. 117.3b, 117.5, 405.2, 603.3, 603.3d, 601.2c, 603.2).
Remember also that the stack doesn't resolve all at once, but rather spells and abilities in it resolve one at a time, and only the topmost spell or ability on the stack can resolve this way (C.R. 117.4).
See also this thread.
EDIT (Jan. 11): Edited.