A resonant long-wavelength infrared (LWIR) absorption filter for thermal detectors is proposed and demonstrated. The filter operates not by transmitting light like a typical Fabry-Perot, but rather by coupling light in a small wavelength region into one mirror where it dissipates and heats the thermal detector plate. The top plate, or mirror, is movable and can be continuously tuned to have a resonance from 8.7 to 11.1µm using electrostatic actuation with 0-42V applied voltage. Since LWIR features are often very course compared to those in the visible, the widths of the tuned resonances are about 1.5µm. At an actuation voltage of 45V, the filter switches into a broadband mode with an absorption width of 2.83µm to enhance sensitivity for scenes with few photons and little spectral information. The switching time between the modes is about 100µsec and the settling time about 400-700µsec. Both times are readily compatible with standard microbolometer frame times.