See if there are updates for your graphics drivers https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

Check your Firefox update settings and make sure Firefox is set to automatically install updates: #Click the Firefox menu button [[Image:New Fx Menu]] and click on <s>Options</s> '''Preferences''' #Choose the '''Advanced''' panel on the left and click on the '''Update''' tab. If you already have Firefox set to update automatically, then the problem could be that your security software is ''intercepting the HTTPS request to download'' the needed components from Mozilla's servers, as reported here: [https://bugzilla.mozilla.org/show_bug.cgi?id=1300633 Bug 1300633] - Users report Netflix stopped working after Firefox 48 as CDM won't get installed <sub> See [https://bugzilla.mozilla.org/page.cgi?id=etiquette.html Bugzilla etiquette] before commenting in bug reports.<sub> Please tell us What security software are you using. You could try to disable the interception of secure connections in your security software settings, as explained in the "Antivirus products" section this article: [[How to troubleshoot the error code "SEC_ERROR_UNKNOWN_ISSUER" on secure websites]]

Correction: On Mac OS, you would click on '''Preferences''' (not Options) to check your update settings.