Firefox will read it if it exists[1]. You could use the /usr/lib/firefox/distribution directory (or whatever the installation directory may be), but that may be overwritten by an update.

There doesn't seem to be any way to set per-user group policies, so unless you're installing firefox in a user-controlled directory, it will require elevated privileges.

[1]: https://support.mozilla.org/en-US/kb/customizing-firefox-usi...