What you're missing is that passwords don't require blobs. The ContentDecryptionModules used for DRM on the Web are blobs.
For a browser to run a blob, there needs to be an interface between the browser and the blob. Flash and Silverlight blobs used a standard interface: NPAPI. Any browser implementing NPAPI could run the blobs (assuming the blob was compatible with the host OS).
CDMs have no such standard browser/blob interface (note that EME relates only to the Javascript environment not the actual browser code). In fact, the only browsers able to run these blobs are the ones that the CDM provider 'trusts' and is willing to provide proprietary information to in order for browser developers to get their blob working. Your independent Firefox fork will almost certainly never be able to support any major CDM; the CDM provider doesn't trust, nor care about your browser.
Therefore, CDM providers arbitrarily _select_ which browsers can have access to their Web content. This is unprecedented. Content on the Web, in whatever form, has always been accessible by _any_ browser; it was merely a matter of that browser implementing the necessary standards. This is the idea of the open Web. CDMs are literally the opposite idea.