Firefox unfortunately does not support the File System Access API. You can try by calling the most fundamental function, the one that asks the user to select a file:

    window.showOpenFilePicker()

What Firefox supports is a "File System API" which creates something similar to the other old ways of storing data in a browser, like cookies and indexeddb. You cannot use it to edit normal files on your disk.

This is my main gripe with Firefox. They force their users to either store everything in the cloud or go through the arcane "download" workflow where you have to manually save files.