It's build its own PDF reader called PDF.js that runs atop the browser engine itself using the JavaScript language. It's already open-source software, like just about everything Mozilla does.

March 11, 2021 /PRNewswire/ -- On March 10, 2021, Accusoft announced the arrival of the free-to-use Accusoft PDF ... build a viewer that's a step above what you can get from open source offerings.