The whole dance has been made significantly easier by the adoption of UF2 flashing by large parts of the custom keyboard hobby: the device temporarily pretends to be a USB storage device, so you can now download the file and drag&drop it to your device.

Still not quite WebUSB-easy, but a massive improvement over needing dedicated programming software!

Firmware updates with UF2 over the emulated mass storage aren't bad, I agree.

But config updates that way still suck. The best implementation I've seen will present you with an empty drive with a README explaining how to drop a uf2 + an editable config file that contains all options with comments.

That's definitely workable for us tech people, but it absolutely sucks for the vast majority of users (including us tech people). Just think about having to learn the syntax, or simple things like picking a color or mapping keys on a keyboard.

IMHO Mozilla should have at least adopted WebSerial. It wouldn't give the entire USB freedom, but it has fewer privacy and security concerns and devices would have make it work. But now it's too late, WebUSB has been adopted widely and Mozilla will eventually have to adopt it or perish.