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-problems-images-not-show http://kb.mozillazine.org/Images_or_animations_do_not_load 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

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