I understand why Mozilla is hesitant. "Why does a browser need to give access to a serial port" is a good question. Certain web tools have definitely proven useful (especially when using an Android device to flash microcontrollers!) but if you asked the average internet user 20 years ago if their browser should provide websites with access to their serial ports, you'd get laughed at.
I hope Mozilla reconsiders their positions on this, because this is just one of those reasons I keep Chrome installed. I need it very rarely, but when I do, it's often because Mozilla made a choice I disagreed with (like their decision to remove anything resembling PWAs on desktop Firefox, which is why I have a bunch of Chrome shortcuts in my application launcher now).