That built in PDF viewer is another feature that could have been an addon. It's bloat which increases the browser's attack surface. It's completely unneeded given that just about every OS ships with some kind of PDF reader out of the box.
AFAICT PDF.js is just another JavaScript application and thus as sandboxed as any other website.
They can be part of the initial install so that Mozilla can provide the browser as they envision it, but be able to be removed for those who have other ideas of what their browser should consist of.
I don't know how technically feasible that is with their code, but it makes sense to me from a developer standpoint.