I have enabled DRM on Firefox and it still didn’t work. I had to enable more repos (the community ones I think) and install libavcodec-extra as per an answer on AskUbuntu.

I’m suspecting the package can’t be preinstalled for licensing reasons (even though I ticked the non-free software box during install) but in this case why not replace the library with a shim that displays a notification to the user and allows them to install the package through a GUI?