Because you run an older version of Mac OS, your Firefox version cannot be updated beyond the October 2021 version. ([[Firefox Mac OS X 10.9, 10.10 and 10.11 users move to Extended Support Release]]) Still, YouTube hopefully hasn't ended support for Firefox 78 that quickly. Let's check a couple things. Could you confirm that DRM (Digital Rights Management) is enabled in your Firefox? More info in the following article: [[Watch DRM content on Firefox]] Sometimes Firefox doesn't keep the Widevine plugin -- needed for DRM video -- up-to-date. In those cases, disabling DRM, waiting 5 minutes, then enabling it again may trigger Firefox to remove and re-download the plugin. Any progress so far?