Ya, it's odd to say it's not on other browsers when they all have it just to variable degrees... it's almost as if there are comment bots here that are trained on outdated data ;)
The ability to access the native filesystem (via showOpenFilePicker()) is indeed only available in Chromium.
For Firefox and Safari users, one can offer a fallback via "download file" and "upload file" functionality. Which is clunky, because it does not reliably remember the directory. So the user often has to navigate to the correct dir again. Also it does not offer to save with the same filename by default. So the user will be offered to download "myfile(2).txt" or something instead. But at least it works and you can tell your users to use Chromium for a better experience.
