User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:76.0) Gecko/20100101 Firefox/76.0

Steps to reproduce:

I recently tried to set Firefox (76.0.1) as my default PDF viewer on Windows 10 due to pdf.js being lightweight and perfectly adequate for my use case.

Edge, Edge Chromium and Chromium already are available to be selected as the default PDF application via Settings -> Apps -> Default Apps. Firefox, however, doesn't seem to register its capability of opening PDF files with Windows at all. Maybe this feature could be implemented in an upcoming FF release?

Actual results:

• Opened Settings -> Apps -> Default Apps on Windows 10.
• Tried to set Firefox as the default app for the PDF file type.
• Firefox wasn't listed as an option at all.

Expected results:

• Firefox should have been listed as an app capable of opening PDF files.

Thanks for filing this bug, lots of good detail here.

We've always intentionally left ourselves out of the list for PDF, but I wonder if it might be a good time to reconsider that decision. Romain, what do you think?

Marking as duplicate of 1506416 since my understanding is that these are the same issues.
We have it in the backlog for pdf.js work, I agree that registering as a handler should be done, hopefully this can be the case for 80.