I found this article and it solved my issue. Let me remember these steps just in case in future it happened again.
in your address bar to get to the advanced configuration page. Click “I’ll be careful, I promise” (and actually be careful because you can break all sorts of things in there if you’re not!) to continue.
to see the options you need.
- Double-click both “media.mediasource.enabled” and “media.mediasource.webm.enabled” in order to change their value to “true”.