The data sent by Firefox to Google is for all visible access points, public or private. For each access point detected, it sends the following data to https://www.google.com/loc/json:
"mac_address": "01-23-45-67-89-ab",
"signal_strength": 8,
"age": 0,
"SSID": "MyAccessPoint"
where
mac_address is the mac address of the WiFi node.signal_strength is current signal strength measured in dBm.age is the number of milliseconds since the WiFi node was detected.SSID is the name or ESSID of the WiFi node.
The georeferenced WiFi data used to geocode your request was collected when Google was driving around taking pictures for StreetView.
It is important to note that, however, that this is how the HTML5 geolocation API is implmeneted on Firefox using Google's Geolocation Services. This is an implementation, not a specification. If your device has a built-in GPS, it is probably desirable that your HTML5 implementation queries the GPS directly, rather than using a geolocation service.
Even firefox implements this feature differently on Windows and Linux. Using a current Firefox on Windows, details of all visible WiFi nodes are sent to Google. Using a current Firefox on Linux, only details of the currently connected WiFi node is sent, due to the reliance on libiw for access point data.
As another answerer noted, Safari uses Skyhook Wireless's service which does much the same thing as Google's Location Services.
