I think the EFF put it right https://www.eff.org/deeplinks/2014/05/mozilla-and-drm "Baker may think that Mozilla cannot change the industry on its own (despite it having done so many years ago). Sadly, it changes the industry by accepting DRM."
Mozilla just told all its users to install IE, Chrome or Safari.
To your second point: If you install IE or Chrome the DRM system will be part of your browser and you have something out of your control running on your system whether you want to watch DRMed content or not.
With Firefox you can just keep using the web, and as long as you never want to watch any DRMed content, nothing changes for you. Only if you want to watch DRMed content will you be asked to download a plugin. As the cherry on top, Firefox will not leak identfiable data about your video usage to DRM or content providers.
Unfortunately Mozilla alone can't stand against the power of Google, Apple, Mircrosoft, Amazon, Netflix and others united in their desire to push DRM through, but if you can't see the difference between Mozilla and the others, I don't think there is anything I can say to change your mind.