Firefox has been loosing market share for years now. As is usually the case with DRM, Mozilla has ultimately been stuck with a loose-loose situation: don't implement DRM and watch helplessly as people complain about their "incompetence", or include closed-sourced, not-free ("free as in freedom") malware (I consider DRM implementations to be over-glorified trojans). There popularity is in trouble either way.
In any case, the real problem is not with those who implement DRM (otherwise there would be age-old complaints against Google, Microsoft, Adobe, etc.), but rather those who demand DRM be used. If Hollywood would just stop spending millions of dollars protecting outdated - not to mention harmful - copyright laws from any form of improvements, this mess wouldn't have happened in the first place. Instead, they're probably celebrating yet another crushing blow to the free and open internet.