Not just that but Firefox has become stagnant when it comes to a lot of interesting Web standards. WebUSB, WebSerial etc... are the ones where people are really pushing the web and Mozilla refuses to implement them.
It is not like the use cases for connecting devices and use them via. the Web Platform will suddenly disappear if they don't support it.
Their reasons to push against WebSerial are not as valid and that would still cover most use cases without the downsides and complexity of the full WebUSB.