https://support.mozilla.org/en-US/kb/fix-common-audio-and-video-issues

An Error Occured http://www.ghacks.net/2014/06/13/fix-error-occurred-please-try-later-youtube-error/

Can your Firefox decode MP4 video? I have a test page (actually, it's a re-post of Google's old test page):

https://www.jeffersonscher.com/sumo/html5-video.html

Ideally you'll have supported in four spots and probably in two spots. If your Firefox has problems with MP4 video, make sure you have FFmpeg installed (at the system level).

i have installed ffmpeg using sudo apt install ffmpeg but still the error persists. My error message occurs when i try to watch you tube live and if i visit any other video platform this problem doesn't occur.. i tried of using google chrome but error doesn't occur in that browser.

my error message: Your browser does not currently recognize any of the video formats available.

Click here to visit our frequently asked questions about HTML5 video.

Does the test page show all positive results at this point?

I generally avoid live streams and watch videos later at 1.25x speed, so I don't know whether they all use a novel encoding, or only specific ones.

If you open the Web Console in the lower part of that tab (Ctrl+Shift+k), it may have a little bit more information about the encoding that Firefox can't play in some error or informational messages.