As I mentioned elsewhere, there haven't been any full time mozilla devs on PDF.js for quite awhile. I'm not sure I actually see the whole maintenance cost savings argument since PDF.js has practically cost Mozllia nothing the last few years. A lot of bug fixes have come from unpaid contributors in that time.

Initially, PDFium was pitched as a freebie if we added support for chromium's flash and then we'd also get improved PDF printing and form support. However, the amount of effort that has gone into supporting PDFium is already far beyond what it would have taken to improve PDF.js form support and help improve Firefox's printing (which would have benefited the web in general). Though, this is my very biased opinion as I was tech lead of PDF.js.