If your wifi interface is enabled, Firefox sends to Google all the wifi signal strength around you in order to locate you (signals strength,mac addresses and locations of all private and public hotspots were captured by Google with their Street View's cars I guess).
Here is the full POST request captured with TcpCatcher (and its SSL mode)
POST /loc/json HTTP/1.1
Host: www.google.com
Content-Length: 4233
Pragma: no-cache
Cache-Control: no-cache
Content-Type: application/json
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.119 Safari/534.16
Accept-Encoding: gzip,deflate,sdch
Accept-Language: fr-FR,fr;q=0.8,en-US;q=0.6,en;q=0.4
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3
{"access_token":"2:fHl0jGoQDeLFKfUn:sn0HJO00jOQV2334","host":"html5demos.com","radio_type":"unknown","request_address":false,"version":"1.1.0","wifi_towers":[{"age":0,"mac_address":"00-13-10-27-ec-ba","signal_strength":-82,"ssid":"linksys"},{"age":0,"mac_address":"00-14-bf-e2-b6-a3","signal_strength":-101,"ssid":"grepre2"},{"age":0,"mac_address":"00-17-33-9e-4d-a8","signal_strength":-94,"ssid":"NEUF_4DA4"},{"age":0,"mac_address":"00-17-33-cc-b5-10","signal_strength":-69,"ssid":"NEUF_B50C"},{"age":0,"mac_address":"00-17-33-cc-b5-11","signal_strength":-66,"ssid":"Neuf WiFi"},{"age":0,"mac_address":"00-17-33-cc-b5-12","signal_strength":-69,"ssid":"SFR WiFi Public"},{"age":0,"mac_address":"00-1a-2b-0e-66-74","signal_strength":-86,"ssid":"NUMERICABLE-AB1B"},{"age":0,"mac_address":"00-1f-33-47-c0-50","signal_strength":-80,"ssid":"SUPERMAN"},{"age":0,"mac_address":"00-1f-33-e0-1e-1f","signal_strength":-80,"ssid":"NUMERICABLE-5866"},{"age":0,"mac_address":"00-1f-9f-f4-1f-17","signal_strength":-90,"ssid":"\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000"},{"age":0,"mac_address":"02-81-31-a2-c4-98","signal_strength":-71,"ssid":"wifi"},{"age":0,"mac_address":"02-81-31-a2-c4-99","signal_strength":-69},{"age":0,"mac_address":"02-81-31-a2-c4-9a","signal_strength":-68,"ssid":"FreeWifi"},{"age":0,"mac_address":"02-81-31-a2-c4-9b","signal_strength":-69,"ssid":"freephonie"},{"age":0,"mac_address":"06-20-9f-00-3d-24","signal_strength":-82,"ssid":"damiconexion"},{"age":0,"mac_address":"06-20-9f-00-3d-25","signal_strength":-82},{"age":0,"mac_address":"06-20-9f-00-3d-26","signal_strength":-81,"ssid":"FreeWifi"},{"age":0,"mac_address":"06-20-9f-00-3d-27","signal_strength":-82,"ssid":"freephonie"},{"age":0,"mac_address":"26-b7-bb-70-e1-d8","signal_strength":-93,"ssid":"DC_baby"},{"age":0,"mac_address":"26-b7-bb-70-e1-d9","signal_strength":-91},{"age":0,"mac_address":"26-b7-bb-70-e1-da","signal_strength":-89,"ssid":"FreeWifi"},{"age":0,"mac_address":"26-b7-bb-70-e1-db","signal_strength":-94,"ssid":"freephonie"},{"age":0,"mac_address":"26-f8-5c-ec-bc-a4","signal_strength":-80},{"age":0,"mac_address":"26-f8-5c-ec-bc-a5","signal_strength":-80},{"age":0,"mac_address":"26-f8-5c-ec-bc-a6","signal_strength":-81,"ssid":"FreeWifi"},{"age":0,"mac_address":"26-f8-5c-ec-bc-a7","signal_strength":-80,"ssid":"freephonie"},{"age":0,"mac_address":"30-46-9a-43-fc-e5","signal_strength":-84,"ssid":"DartyBox_98DE"},{"age":0,"mac_address":"32-46-9a-43-fc-e6","signal_strength":-84,"ssid":"DartyBox_98DE_WEP"},{"age":0,"mac_address":"4e-0a-94-3d-d6-38","signal_strength":-82,"ssid":"neko"},{"age":0,"mac_address":"4e-0a-94-3d-d6-39","signal_strength":-82},{"age":0,"mac_address":"4e-0a-94-3d-d6-3a","signal_strength":-80,"ssid":"FreeWifi"},{"age":0,"mac_address":"4e-0a-94-3d-d6-3b","signal_strength":-82},{"age":0,"mac_address":"5c-33-8e-eb-6c-c0","signal_strength":-87,"ssid":"Livebox-0510"},{"age":0,"mac_address":"72-f5-aa-cd-b7-e4","signal_strength":-86,"ssid":"Kermazeguen"},{"age":0,"mac_address":"72-f5-aa-cd-b7-e5","signal_strength":-87},{"age":0,"mac_address":"72-f5-aa-cd-b7-e6","signal_strength":-87,"ssid":"FreeWifi"},{"age":0,"mac_address":"72-f5-aa-cd-b7-e7","signal_strength":-88,"ssid":"freephonie"},{"age":0,"mac_address":"76-a6-a6-5a-90-c8","signal_strength":-78,"ssid":"pipiou_wifi"},{"age":0,"mac_address":"76-a6-a6-5a-90-c9","signal_strength":-79},{"age":0,"mac_address":"76-a6-a6-5a-90-ca","signal_strength":-77,"ssid":"FreeWifi"},{"age":0,"mac_address":"76-a6-a6-5a-90-cb","signal_strength":-78},{"age":0,"mac_address":"86-9c-8f-a0-ac-e4","signal_strength":-78,"ssid":"touhere"},{"age":0,"mac_address":"86-9c-8f-a0-ac-e5","signal_strength":-82},{"age":0,"mac_address":"86-9c-8f-a0-ac-e6","signal_strength":-83,"ssid":"FreeWifi"},{"age":0,"mac_address":"86-9c-8f-a0-ac-e7","signal_strength":-82,"ssid":"freephonie"},{"age":0,"mac_address":"c0-3f-0e-b6-9a-37","signal_strength":-64,"ssid":"NUMERICABLE-825A"},{"age":0,"mac_address":"da-0f-67-af-23-10","signal_strength":-71,"ssid":"freeboxtomseb"},{"age":0,"mac_address":"da-0f-67-af-23-11","signal_strength":-70},{"age":0,"mac_address":"da-0f-67-af-23-12","signal_strength":-71,"ssid":"FreeWifi"},{"age":0,"mac_address":"da-0f-67-af-23-13","signal_strength":-71}]}