The evolution of plasma ejected earthward from a patch of reconnection at ~25 RE is studied using a double-adiabatic-MHD simulation of a thin filament. Firehose instability occurs in the simulation after a compressional shock reflects from the near-Earth region. The tailward-propagating compressional wave, which brakes the earthward flow in the filament, is characterized by strong magnetic fluctuations. Within the context of the Near-Earth-X-Line model of substorms, we suggest that firehose instability might cause the intense magnetic-field fluctuations observed in the inner plasma sheet at substorm onset.