Was it working before? Is this with all websites/pages, some, a few? If you have Windows 10 N or KN then you may not have the Media Feature Pack installed. https://support.mozilla.org/en-US/kb/fix-video-audio-problems-firefox-windows https://support.mozilla.org/en-US/kb/fix-common-audio-and-video-issues https://support.mozilla.org/en-US/kb/html5-audio-and-video-firefox https://support.mozilla.org/en-US/kb/troubleshoot-issues-with-plugins-fix-problems https://support.mozilla.org/en-US/kb/warning-unresponsive-plugin