Cannot play media. No decoders for requested formats: video/mp4

Suddenly, most videos won't play. Upon closer inspection of the console messages, I get the following: ''' "Cannot play media. No decoders for requested formats: video/mp4; codecs="avc1.42E01E mp4a.40.2", video/mp4; codecs="avc1.42E01E mp4a.40.2", video/mp4; codecs="avc1.42E01E mp4a.40.2", application/x-mpegurl, video/mp4; codecs="avc1.42E01E mp4a.40.2", application/x-mpegurl"''' Only on Firefox, I can play mp4 videos normally in other browsers and media players. Tried reinstalling Firefox, refreshing, restarting the browser and the computer... My about:config media.mp4.enabled is set as "TRUE". Other messages from the console: "[warn][id_playback] id playback error: Object { code: "DNS", description: "Device not supported", reportAs: "INFO", internalError: true } player.min.js:1:55026 [error][player_error] <empty string> Object { description: "Device not supported"" Also tried disabling hardware acceleration and restarting the browser. I'm clueless!