What the OpenH264 Plugin is used for https://support.mozilla.org/en-US/kb/open-h264-plugin-firefox

If you are just trying to play a video file on Windows then you may need a media player like say VLC https://www.videolan.org/

Also if you are not located in say Canada or USA then you may have a N or KN edition of Windows 10. https://support.mozilla.org/en-US/kb/fix-video-audio-problems-firefox-windows

Thanks James for your reply. Your comments made me recall something I should have posted originally. When I opened the video file on Windows, the "app" that starts "automatically" is Movies & TV (from Microsoft). I also tried playing the file using Windows Media Player, but that also did not allow the playback. I am located in Canada and/or USA depending upon my travel. I understand Open H264 is for streaming. So, why is the Movies & TV app telling me I need a new codec, as per image originally posted? PS. I have not yet tried videolan

The HEVC codec is H265 which requires a license. Go to about:support in the address bar and check the "Codec Support Information" section. Your GPU may have a license for HEVC hardware decoding but not software decoding, check that you have hardware acceleration enabled. To use it you will need to set media.wmf.hevc.enabled to true in about:config then restart the browser.

VLC can play HEVC, just maybe not 4k smoothly. You could also try the "MPV .net" media player.