Unfortunate clash of acronyms:

> 	Please enable playback of DRM protected content.

This is DRM in the sense of Digital Rights Management

This depends on proprietary blobs obfuscating the secrets required to
decrypt the content in order to prevent you from accessing the content
directly. This can't be open source for rather obvious reasons.

> In Firefox's about:config I see "drm" and "DRM",

Not sure what that is, but this almost certainly related to the 
Direct Rendering Manager aka drm(4).