Why is that true? Some applications require file system access. People would grant the application the permission to access files like any other resource like camera, mic, location.
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.