I don't think DRM really prevents anyone from watching video. There are plenty of alternative options. For example, for popular films, one option would be not to use the streaming video service but instead purchase a DVD. Another alternative is to use a platform that supports the desired DRM-laden streaming video service. A more difficult approach would be to reverse engineer the DRM software and reimplement it for the chosen unsupported platform. So it's not ultimately limiting, at most it's a small inconvenience.

The same arguments apply to the issue of backups. It would be highly unusual for the only copy of a media product to be only accessible via a DRM-encumbered streaming video service.

I do see your point on the lack of trust though. In this case, hopefully Mozilla's sandboxing approach would go some way towards mitigating that. And the inevitable disassembly of whatever closed-source binary they release.