In Firefox, the API location can be overidden like in Chrome. In this case, you have to rely on the Geolocater addon. Install it from the official Mozilla addon website.
After installation, go to "Tools" menu, "Geolocater", "Manage". From here, you will be able to directly add a "fake" geolocation by browsing a map or searching place. After, you will be able to override geolocation and switch between predefined geolocations. You can confirm the behaviour, going to the html5demos for Geolocation