There's nothing a PDF can do that a webpage can't. In fact there are a hundred of things that a webpage can do, but a PDF can't. Including, form fields, input fields and seamless form submissions.
Webpages can also do this: https://bubblin.io/cover/official-handbook-by-marvin-danig#f...
Disclosure: It's my work.
PDFs are easy to make and easy to work with. Web pages aren't.
Your work is impressive, and why would anyone want that? Do you envision lawyers putting all their legal contracts into fancy flippy books?
Someone will have to solve it for the lawyers in a not so 'fancy consumerish' way. Point is that it is possible to do that, and Firefox shouldn't be solving this problem using an ancient format and a layer of cruft in between.
You can be a developer who enjoys the smell of your own farts all day long but that doesn't mean anyone else wants to smell them.