AFAIK the plugin interface for Flash is something generic, its not made for Flash only. This DRM plugin will have open-source code written specially to support DRM plugins, and thus Firefox/Mozilla has given up, and actually endorses the W3C EME, and endorses DRM. That is bad. I'd rather use a browser that doesn't endorse that.

I think the EFF put it right https://www.eff.org/deeplinks/2014/05/mozilla-and-drm "Baker may think that Mozilla cannot change the industry on its own (despite it having done so many years ago). Sadly, it changes the industry by accepting DRM."