I had basically the same problem when I updated FF three days ago. SMDB would not display at all, and YouTube videos would not start playing. I reinstalled twice with the same result.
Then I “refreshed” FF, which removes plug-ins. I had been using AdBlock. Without AdBlock, FF started right up, with all my bookmarks and passwords, but with no history at all. I then installed uBlock and it still works fine.
One odd thing I noticed was this: Prior to the update, if I started a YouTube video with the sound muted, I could not unmute it. I would have to reload the video again. With the newer “refreshed” installation, the problem disappeared. I can mute and unmute as much as I care to. Go figure.