Firefox tried to push open video formats, like webm, and refused to support H264... yet, after years of fighting they gave up, mostly because MS and Apple refused to support it to push their (patented group) H264 format. Only if google switched youtube to webm and stopped supporting H264 it would be possible to do something like that, but even if the webm was a google format, they never really pushed that change and H264 won this round.
Future video support is the new battleground. Yet W3C is set to accept DRM and firefox not supporting it would mean that important sites would either push the usage of other browsers (like netflix) or push the installation of broken plugins (like all the silverlght sites we have today) that may just exist in windows. Either way firefox would be lonely on this battle, as MS, Apple and Google all have interest in DRM video, so it would be a lost battle from the start. It is sad, but delivering video is only set to increase and big companies want to make money from it... even if the browser would not have any DRM, they would create "apps" to support it so that movie industry would allow online video streaming. It is a lost battle, since there is demand for it, not from the users, but from the content makers... and we all know they are stupid, they prefer having no market (and so piracy) than provide open access to their content, just look how music industry works with the internet and how long that battle is taking place
Firefox solution is to use a Adobe "plugin" that is very restricted on what it can do (read a stream, reply a stream), just to decode the DRM. This would allow the DRM validation that some companies require, allow one to disable this very easily and allow for future replacement of that closed "plugin" with any other open implementation (trying to push directly a open DRM "plugin" now could blacklist firefox if someone tried to remove the protection... later, with existent market share it would be harder to blacklist firefox)
So yes, no one wants DRM, not even Mozilla, but looking at the alternative (some other DRM support or protocol you can't control), at least Mozilla can have some control and impose limits by doing this and not sacrifice market share on a battle that would be lost anyway. Don't blame Mozilla on this one, blame MS, Apple and Google for teaming up pushing DRM, so much that W3C have also agreed to add a DRM standard.