Both of these molecules experience dipole-dipole forces, but since the bromine atom (Br) is much bigger than the chlorine atom (Cl), HBr is, therefore, the larger, more polarizable molecule compared to HCl. This makes the IMFs in HBr stronger than that those in HCl, making HBr have a higher boiling point because stronger dipole-dipole forces = higher boiling point.
The same can be said here. Br2 molecule is larger and is thus more polarizable than the Cl2 one. This makes the former have stronger dispersion forces (take note that these molecules are nonpolar due to two atoms of the same identity bonded to one another which then means that they experience dispersion forces instead of dipole-dipole). Stronger dispersion forces = higher boiling point.
