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.
I would not call it unfortunate. Browsers should not have access to the file system, USB ports, etc. Perhaps in "PWA"s, if the user has a way of giving and withdrawing consent.
I think the flow of Chrome where the user has to select a file from disk and then acknowledge a prompt that the site will have access to that file until they close the window is fine. But if Mozilla wanted to protect users from themselves even more, they could make it as strong as they like. For example, by requiring to turn the functionality on in about:config.
Not giving the user the choice at all, if they want a cloud only browser or a browser that lets them work with local files is the wrong way.