ffmpeg -i firefox.windows10.test.webm

Input #0, matroska,webm, from 'firefox.windows10.test.webm':
  Metadata:
    encoder         : QTmuxingAppLibWebM-0.0.1
  Duration: N/A, start: -0.001000, bitrate: N/A
    Stream #0:0(eng): Video: vp8, yuv420p(progressive), 640x480, SAR 1:1 DAR 4:3, 1k tbr, 1k tbn, 1k tbc (default)
    Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)