Geolocation provider for Firefox that allows manual input

This my Answer to the stackoverflow question: Geolocation provider for Firefox that allows manual input:

The geo.wifi.url does not need to be a webservice. You can also set it to a local uri with file://...

The file should be a json file with content like this:

{"location": {
              "latitude": 48.777025000000002, 
              "longitude": 9.1713909999999998, 
              "accuracy": 10.0}}

Update: For Firefox 9+, the format has changed:

{
    "status": "OK",
    "accuracy": 10.0,
    "location": {"lat": 48.777, "lng": 9.171}
}

Or you can combine them to support both:

{
    "status": "OK",
    "accuracy": 10.0,
    "location": {
        "lat": 48.777,
        "lng": 9.171,
        "latitude": 48.777,
        "longitude": 9.171,
        "accuracy": 10.0
    }
}

Update: it looks like manually setting this preference is blocked by the provider assuming the Google service. See Bug 716453