[sowbug]

You could do that, but then either you're asking users to install a native app, which isn't very "webby," or you're shipping something with Firefox that isn't based on web standards*, which absolutely isn't webby.

*Unless you're thinking of something I'm not, the API couldn't exactly match the Web Serial API because of same-origin policy, and if you made an exception to that policy to make the polyfill work, you'd punch a giant security hole in the browser.