Back then, Mozilla's plan was for the CDM to inspect the running Firefox and deny playback if it detects tampering of the relevant browser parts.
How the hell that is supposed to work if the CDM is oh-so-sandboxed is left as an exercise for the reader.
Did anyone actually check that own builds of vanilla Firefox sources are able to use the Adobe CDM? Wouldn't surprise me at all if it only worked with the official Mozilla-signed Windows binaries (yep, at least the Mozilla flavour of the "portable" and "interoperable" EME is provided for Windows and Windows only).
