There are two major limitations on doing this: 45ESR, on which TenFourFox is based, has only an incomplete EME implementation, and then the CDM needs to be written. There are some open-source Widevine implementations that could be adapted but the work to do this is non-trivial.
The bottom line is that it's probably possible, but it's more work than I can personally do right now.
That's too bad, but I appreciate you explaining the limitations here. I think this would be a great thing to eventually have come to TFF, although I have concerns about performance, especially on the iBook G4 I use.