geo.provider.use_corelocation: true/false # presumably for tracking on MacOS
geo.provider.use_geoclue: true/false # presumably for tracking Linux users with Geoclue2 provider [1]
geo.enabled: true/false # presumably, turns the whole thing off
Some say[2][3], use_ options take precedence over network.url, so you need to set those to false.
It also appears[3][4], that setting geo.provider.testing to true might be required.
user_pref("geo.provider.network.url", 'data:application/json,{"location": {"lat": 45.0, "lng": -122.0}, "accuracy": 128.0}');
I _believe_ this also stops wifi data from leaking anywhere.