By the looks of it they use their own player which uses HTML5 or site can choose which one it wants to use. It could be one of your Extensions, uBlock Origin has been having some issues with that. Or your Profile maybe... Please : https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer and https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles then https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings TEST....... If no issues then Extensions which need to be added back in 1 at a time and tested ..... Or it is your Profile : Make a new one and test ...: https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles Please let us know if this solved your issue or if need further assistance.