Web usb and serial are not just missing, last I checked Mozilla is opting to not implement based on their moral stance. It just puts them behind for some stuff.
Half the thread impunes Mozilla for taking so much money from Google and imply that they are controlled opposition, and the other half gets upset when Mozilla doesn't implement every standard that Google tries to steamroll through the standards bodies because of objections to how they can be used for fingerprinting, or complains that the attempts at anti-fingerprinting break websites, etc.
Sometimes it's not even different people, it's the same people punching them for contradictory reasons.
Mozilla is not perfect but they get all the downsides of being methodical and privacy focused alongside none of the benefits. Everybody hates the "side projects" unless it's Rust, Servo, LetsEncrypt, Thunderbird, contributions to Opus/AVI, etc. and you can be sure they'll be criticized if they "focus" by touching investment in any of those by the same people.
Yeah, double standards at its max. Firefox inputs every privacy concerns for these APIs that Google puts 0 Vietnam Dong to care about users' privacy. And those people cry about why Firefox doesn't implement it.
Yeah, it really undermines their ability to compromise user security and privacy.
Somebody also recently shared an awesome project which let's you use an usb printer regardless of your OS driver.
I used something similar in the past. It was a legitimate use case for web usb which changed my mind on it quite a bit.
https://www.charachorder.com/ sells ergo keyboards and allows you to update their firmware directly in the website, through web usb. No local apps at all. Also an improvement in overall security from having to download some .exe / .dmg and running it locally.