Nice to see Firefox still trying to innovate. The UI looks good. I would prefer something like that, implemented in a similar way to PDF++, where the comments and highlights are stored in a markdown file.
That’s fair; still, I heavily rely on embedded annotations when I review things for other people.
I really do like that PDF++ supports embedded annotations and has a way to link to them.
However, creating these embedded annotations in PDF++ is still quite annoying, as the whole PDF has to be rerendered when anything (e.g. the comment) changes. I hope this improves with the official pdf.js support for annotations with comments.
New library that might be of interest to replace pdf.js, maybe?: LibPDF-js/core: A modern PDF library for TypeScript. Parse, modify, and generate PDFs with a clean, intuitive API.
(Or augment, at least for a time being. Robust modification, but no rendering yet.)
Looks like pdf.js is ready to integrate annotation with now 
Hopefully the team can prioritize this next
+1 This is badly needed
is there any update on when this might materialize?
I need this so much