Please explain the problem in detail. What happens? Error messages?

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

My problem is shown in the following image.

Your browser does not currently recognize any of the video formats available Click here to visit our frequently asked questions about HTML 5 video

See if there are updates for your graphics drivers https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

Do you have any special YouTube extensions installed?

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems