Re: Unfortunately, your browser does not support video playback.
It looks as though I can verify VMC's problem.
I have a full install of Xubuntu installed in a VM, and fully updated. I can play non-drm .mp4 files in Firefox on it.
I also have a minimal install of Ubuntu in a VM, which won't play the same .mp4 files in Firefox. Thinking that maybe the minimal install was the problem, I tried the latest live ISO, which also won't play these files. It just offers to download them.
(The files are in a local website on my hard drive, which I have for testing purposes.)
BACKUPS are unsexy — until you discover you should have done one yesterday.
Spare your nerves and do one before you upgrade or install.