You have to explicitly grant permission for a site to use a serial port.
Unfortunately, Mozilla decided that this (and other related functionality) is "harmful". https://mozilla.github.io/standards-positions/#webserial
It is a shame, because the overlap between people who use Firefox as their main browser, and people who tinker with microcontrollers is likely pretty large.
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).
Why does a program need to give access to a serial port?
> 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
What if you included "Only if you allow it"?
> What if you included "Only if you allow it"?
You'd probably hear something like "IE/Opera is bloated enough already", I just want my downloads to finish faster.