Web player not working in Firefox 61.0.1 (Ubuntu 18.04)

[whoever]

I get the "Enable player in your browser" message even though DRM is enabled and the computer has been restarted since enabling it.

Firefox 61.0.1, Ubuntu 18.04

[BiggityBoss]

I have a solution for your problem. It is likely you didn't check the "Enable Restricted Formats" box while installing Ubuntu. There are codecs you need to install, and it's done with a single terminal command:

sudo apt update && sudo apt install ubuntu-restricted-extras

It installs all of the media codecs needed by the Web Player and a few other apps on Ubuntu. I just installed them, and then closed and reopened Firefox, and it worked. 
If this doesn't work, consider restarting Ubuntu after installing the codecs.

For more information, refer to where I got this information from (NB: I'm running 18.04, so this is still relevant): http://ubuntuhandbook.org/index.php/2017/10/install-media-codecs-ubuntu-17-10/

Hope this helps.