  We revisit supernova (SN) bounds on a hidden sector consisting of
millicharged particles $\chi$ and a massless dark photon. Unless the
self-coupling is fine-tuned to be small, rather than exiting the SN core as a
gas, the particles form a relativistic fluid and subsequent dark QED fireball,
streaming out against the drag due to the interaction with matter. Novel bounds
due to excessive energy deposition in the mantle of low-energy SNe can be
obtained. The cooling bounds from SN~1987A are unexpectedly not affected in the
free-streaming regime. The inclusion of $\chi\bar{\chi}\rightarrow \rm SM$
substantially modifies the constraints in the trapping regime, which can only
be treated hydrodynamically. Our results can be adapted to generic sub-GeV
self-interacting dark sectors.
