I haven't tried that add-on.

Is the problem with playing the saved MKV file in Firefox? How are you doing that? When I try to open an MKV in a tab, Firefox wants to save it back to disk again.

Do other programs on your system play both the audio and video in the same MKV file?

ANT.COM downloads the selected file to the computers hard drive. I'm using a windows 10 laptop. When i open the file i have tried windows media player, divx, nero and real player. Same results, video but no audio. As i indicated, if the downloaded file is mp4, no problem. I goggled the problem and found if you install VLC media player the MKV file will work. I installed VLC and i do get video and audio. Checking Firefox options/applications you find for MKV and MP4 Firefox uses a VLC web plugin to play the file. It works for MP$ but not MKV.

From your last post, I can't tell whether VLC is the solution, or VLC only works outside of Firefox and the VLC plugin is still mute when trying to play the MKV inside Firefox.

If the plugin is not working, does Firefox detect that the plugin is sending audio? Usually if Firefox thinks a tab is playing audio, a little speaker icon will appear on the tab.

VLC I referenced is a 3rd party application from VideoLan. It does open the MKV file. If i want to put the MKV file on a flash drive to play on another device, video but no audio. That is the issue i need an answer. The Firefox VLC plugin for playback of installed MKV file doesn't work.

While the VLC plugin (in Firefox) is playing the video, does Firefox detect that the plugin is sending audio? Usually if Firefox thinks a tab is playing audio, a little speaker icon will appear on the tab.

I wonder why the video is saving as an MKV file. Are those WebM format videos when they are playing on the site? Usually the format information is available if you right-click the video and check the "Statistics" (toward the top of Firefox's context menu for HTML5 video) or on YouTube's menu I think it says something about data for nerds.

I checked the video as it was playing in You Tube. Mime type-video/mp4. After download using 3rd party ANT.COM. The file type on saved file was MKV. I compared You Tube videos as they were playing after download the file formats were different., MKV and MP4 statistics: MKV: <iframe width="854" height="480" src="https://www.youtube.com/embed/1MCarkGd4Xk" frameborder="0" allowfullscreen></iframe> MKV: mime type video/mp4 coded "avci.4d401e" dash yes 135/251 MP4: <iframe width="854" height="480" src="https://www.youtube.com/embed/B4J4SOO73B0" frameborder="0" allowfullscreen></iframe> MP4: mime type video/mp4 codecs dash yes 135/140 Maybe the problem is ANT.COM .

Hmm, if you are playing a saved file, it's odd that there is an iframe with a web URL. Clearly I don't know what's going on.