  The subsonic expansion of bubbles in a strongly first-order electroweak phase
transition is a convenient scenario for electroweak baryogenesis. For most
extensions of the Standard Model, stationary subsonic solutions (i.e.,
deflagrations) exist for the propagation of phase transition fronts. However,
deflagrations are known to be hydrodynamically unstable for wall velocities
below a certain critical value. We calculate this critical velocity for several
extensions of the Standard Model and compare with an estimation of the wall
velocity. In general, we find a region in parameter space which gives stable
deflagrations as well as favorable conditions for electroweak baryogenesis.
