WebSerial was just introduced in Firefox 151. It was already available for 5 years in Chromium based browser. It's so new in Firefox that even caniuse is not up-to-date: https://caniuse.com/web-serial .
That makes me curious. It would not be quite as seamless as having serial support included out of the box in the browser, but couldn't you get most of the way there by writing a native application that provides provides a network interface to the serial ports and then a JavaScript library for use in the browser than talks to that application over the network (maybe even making the JavaScript library API match the Web…
*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.