Quick summary.

Firefox does not support talking to arbitrary USB devices (whereas Chrome has WebUSB to do that). However, Firefox does support talking to U2F security keys over USB.

This project programmed a microcontroller to pretend to be a U2F security key. The goal was to talk to the microcontroller over USB through Firefox.

They use the Javascript Credentials APIs (https://developer.mozilla.org/en-US/docs/Web/API/Credential_...) and some cleverness to send data to and receive responses from the microcontroller.