They're going to have to implement this sooner or later if they want to remain relevant. More and more happens in the browser, regardless of whether they like it or not.
Yes, as a FF user it’s becoming more and more clear that FF is falling behind on implementing new APIs. It’s frustrating having to keep Chrome around to load certain web apps.
This seems like the perfect thing to implement as a browser addon. If you think you need it, install it. Otherwise, the code doesn't touch your install.
Much like the container tabs extension, in terms of requiring an addon for certain functionality. Or, if mozilla doesn't do it themselves, a (horrifying though warning) third-party-extension with an external helper application to connect to the serial device, and a API polyfill injected into every page.
Consider:
> Show HN: Trick people into flashing their ESP32s with your firmware by visiting your web page