So time to fork Firefox?
It seems to me that refusing to ship DRM makes sense if it gives leverage against the content companies, such that could be used to make them adopt DRM-free solutions. To get such leverage it would be necessary both that the reach of the unencumbered web was compelling and that requiring DRM would significantly limit that reach. Unfortunately plugins have historically provided a DRM-enabled way of distributing content via the web, so there was no compelling reason for content makers to forgo DRM in the past. This makes it significantly more difficult to change the industry today.
It is possible that there was a chance to win back some ground with the decline of plugins on mobile. However, it's also possible that the prevalence of native apps on those platforms made the web itself a less appealing medium for distributing content to mobile users. In any case, the eagerness of Microsoft and Google to deploy in-browser DRM means that we didn't get to run that experiment. Instead we are in a situation where some high percentage of web users can get DRM encumbered video content in their current browser. For those who can't the barrier to switching browsers is typically rather small. Not shipping the capability to view DRMed video in this situation is, tragically, trying to hold a non-equilibrium position; it might be principled but it is going to end in a significantly sub-optimal outcome.
I suppose the optimistic view is that video will follow the iTunes model and DRM will be a short term burden that we have to bear until technology and attitudes in the industry make it clear that there's a better way. I'm confident that Mozilla will continue to push to find that better way.
* Assuming one that exists that is compatible with your OS + hardware + etc.; all the usual problems of binary blobs in general and DRM in particular still apply, of course.