[ayumi]

Does the esptool.js stuff only work in chrome based browsers? Doesn't seem to trigger the serial port dialogue for me in Firefox

IIRC Mozilla deliberately didn’t implement WebUSB and WebSerial, so there is no way to access an USB device from a website’s JS, unless you are using WebMIDI, which was implemented by Mozilla. The fact that they implemented WebMIDI at all may suggest that they have changed their stance and one day they will also implement WebUSB and WebSerial. For now though you will need to use a Chromium–based browser if you want esptool.js to work.