[Ileca]

I didn't have to turn this on. It's in Content section by default...

https://support.mozilla.org/en-US/kb/enable-drm

media.eme.enabled to false to disable EME if you want to tweak the config.

[majewsky]

I had to turn it on: FF 45.x-esr on OS X (cannot specify ".x" because apparently the "About Firefox" dialog is broken).

[CaptSpify]

I had to turn it on: 45.0.2 on Linux