You can examine some code via the resource:// URI (viewer.js):

• view-source:resource://pdf.js/
• view-source:resource://pdf.js/build/pdf.js
• view-source:resource://pdf.js/web/viewer.js

• https://github.com/mozilla/pdf.js

Are you applying security to your PDFs (e.g., no copying)? Except for the password to open the PDF, the other security settings are on the honor system, and the pdf.js viewer may disregard them.

Firefox PDF Viewer isn't a complete viewer, yet. It started an an experiment - PDF.JS add-on - to create an open source PDF viewer based upon JavaScript / ECMA Script. https://en.wikipedia.org/wiki/ECMAScript

IMO it was added to Firefox as a standard feature too early - it should have remained an add-on for a longer period of time, like until it was more mature. And "as things sometimes happen" the original developer of JavaScript was forced out of Mozilla (in my opinion, that is) before PDF.JS development was anywhere near complete. And iirc that was about the time (or right after) it became a standard feature in Firefox. I have seen mention that Mozilla will be replacing that PDF.JS feature in the near future with an open-source package that has wider usage, like with another web browser. I don't recall where I read that, nor do I recall a time frame for when that may happen.

For an individual user I would recommend using a "free for personal use" PDF viewer, but for an "organization" there may be "seat license" fees involved.

the-edmeister said

I have seen mention that Mozilla will be replacing that PDF.JS feature in the near future with an open-source package that has wider usage, like with another web browser. I don't recall where I read that, nor do I recall a time frame for when that may happen.

It looks like work is in process to use the PDFium library used in Chromium-based browsers, which is based on technology contributed by Foxit Software. It's not clear it will become the default, or when that might be decided.

See also:

• https://wiki.mozilla.org/Mortar_Project