There is a error or 2 in you your Video Card Drivers. I do not do Mac so is there a way to just reinstall them..... https://support.mozilla.org/en-US/kb/html5-audio-and-video-firefox 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 Try https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode and test. If can play, possible issues with your Profile Create a New Profile https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles and test. Please let us know if this solved your issue or if need further assistance.