According to MDN the Origin private file system is supported from Firefox 111 but I'm using Firefox 120 and the function:
window.showOpenFilePicker
is undefined. Why is that? Was it removed from Firefox? Or do I need to setup something to use this API?
The showOpenFilePicker method is part of the File System Access API, which is not a WhatWG or W3C standard, and not implemented by Firefox. The File System Access API extends the WhatWG standard File System API (a.k.a "origin private file system"), which is fully supported by Firefox. Mozilla considers the File System Access API partly harmful, thus an implementation is unlikely to happen in the near future.