This is in the works. It's the same codebase as the desktop version, which is written in js.

I've tested it on low end Android phones from 2017, it works well, when compared to the native offering, and also on modern high end phones, it also works well of course.

What's missing is mobile specific UX, such as pinch-to-zoom, and UX/UI refinements, so it's nice to use.

It has form support, annotations and other goodies like the desktop version as well, which I find very useful.

about:config in Firefox for Android Nightly,pdfjs.disabled to false to give it a go.