I don't think Metcalfe's law applies here - people don't use encrypted HTML video as a communication network, it's mostly a channel from content producers to users. So its value only grows linearly with the number of users that can receive it.
More importantly, though, content is more compelling than browsers, and browsers are easier to switch. If Firefox blocks its n users from receiving some content, some fraction of those will leave Firefox and get the content anyway, leaving Firefox with less strength to fight the next battle. In this case, it looks like that fraction would be pretty significant, and the impact on DRM not that great.
More importantly, though, content is more compelling than browsers, and browsers are easier to switch. If Firefox blocks its n users from receiving some content, some fraction of those will leave Firefox and get the content anyway, leaving Firefox with less strength to fight the next battle. In this case, it looks like that fraction would be pretty significant, and the impact on DRM not that great.