As the topic says, this is about sharing everything (scripts, settings, ...) between multiple browsers, f.e.: chrome and firefox. So if I add a script in firefox, it should be automatically availiable in chrome. If I edit a script in chrome, firefox should receive the edit.
Is this possible?
May be not out-of-the-box, but with sym/hardlinks of the extension directory?
Unfortunately there is no way to (automatically) sync the settings at the moment. However, you can export them and then import them manually.
Besides the manual ZIP-based sync: no.
Extensions do have very (very very) limited file system access. They can only write files (download) to a unknown (because browser- or user-defined) folder and read (but not list) files (except Firefox and MS Edge) if local file access is granted by the users. Sorry.
This depends on where you've saved it. Depending on you browser settings it might be your Downloads folder or whatever folder you've configured or you've selected.