We consider premixed gaseous flames propagating between parallel plates (Hele-Shaw cell) and qualitatively analyze how the resulting losses of momentum and heat affect the flame stability for wavelengths of wrinkling that noticeably exceed the plate spacing. Modelling the flame as an effective interface and using Euler-Darcy equations for the gas on both sides, we firstly show that friction to adiabatic walls modify the Landau-Darrieus instability in two ways: a damping coefficient is brought in, along with a Saffman-Taylor type of instability. Being due to friction-induced pressure gradients, the latter may even dominate the stabilizing influence of gravity for downwardly propagating fronts in narrow channels.