  We consider front propagation in a reactive Boussinesq system in an infinite
vertical strip. We establish nonlinear stability of planar fronts for narrow
domains when the Rayleigh number is not too large. Planar fronts are shown to
be linearly unstable with respect to long wavelength perturbations if the
Rayleigh number is sufficiently large. We also prove uniform bounds on the bulk
burning rate and the Nusselt number in the KPP reaction case.
