Ambient guitars. Many of them. Strange, uncommon harmonies that sound uncannily familiar. Multiple acoustic guitars, all recorded by Fred Baty (aka Crepusculum), create a rich soundscape. Rhythmic chords and melody patterns render percussion unnecessary.
Fledgling Firework has it all: soft guitars; calm intro; a slow build up for the final chorus; final chord. End. All is well.
For Hannah slowly gains more and more voices, culminating in a rich finale in minute four. Acoustic post-rock, so to say. The final riffs sound pretty satisfying! Just a fade out ends the song kind of abruptly.