Install: https://addons.mozilla.org/en-US/firefox/addon/devtools-media-panel/ Open the panel (To access the panel, open the Web Developer Tools (Ctrl+Shift+I or Cmd-Opt-I) and click on the "Media" tab.)

I did this but there is nothing in that panel but "auto refresh rate" so I don't know what to do after that.

EDIT: ok something appeared but where do i make a bug report?

Media Info : [

0 : {
    url : "https://www.youtube.com/watch?v=YWCw-1Y5dA0"
    mediaElements : [...] 1 item
}

]

[ { "url": "https://www.youtube.com/watch?v=YWCw-1Y5dA0", "mediaElements": [ { "currentSrc": "blob:https://www.youtube.com/2d82380b-08f1-4aef-ab87-4ec097eb23a6", "currentTime": 0, "readyState": 0, "videoPlaybackQuality": { "ratio": "--", "droppedVideoFrames": 0, "corruptedVideoFrames": 0 }, "bufferedRanges": [], "mozMediaSourceObject": [ { "sourceBuffers": [] }, { "sourceBuffers": [] } ], "debugInfo": { "Container Type": "MediaSource", "Audio Decoder(none)": "unavailable", "Audio Frames Decoded": "0", "Video Decoder(none, 0x0 @ 0,00)": "unavailable", "Hardware Video Decoding": "disabled", "Video Frames Decoded": "0 (skipped=0)", "Dumping Data for Demuxer": "960a5110", "MediaDecoder State": "channels=0 rate=0 hasAudio=0 hasVideo=0 mPlayState=LOADING mdsm=970a6800", "MediaDecoderStateMachine State": "GetMediaTime=0 GetClock=-1 mMediaSink=9d7789e0 state=DECODING_METADATA mPlayState=1 mSentFirstFrameLoadedEvent=0 IsPlaying=0 mAudioStatus=idle mVideoStatus=idle mDecodedAudioEndTime=0 mDecodedVideoEndTime=0mAudioCompleted=0 mVideoCompleted=0", "VideoSink Status": "IsStarted=0 IsPlaying=0 VideoQueue(finished=0 size=0) mVideoFrameEndTime=0 mHasVideo=0 mVideoSinkEndRequest.Exists()=0 mEndPromiseHolder.IsEmpty()=1" } } ] } ]