We show that if Y is the boundary of an almost-rational plumbing, then the framed instant on Floer homology I# (Y) is isomorphic to the Heegaard Floer homology y HF (Y ; C). This class of 3-manifolds includes all Seifert fibered rational homology spheres with base orbifold S2 (we establish the isomorphism for the remaining Seifert fibered rational homology spheres—withbaseRP2—directly). Our proof utilizes lattice homology, and relies on a decomposition theorem for instanton Floer cobordism maps recently established by Baldwin and Sivek.