Some modern websites offer services which are quite convenient to use if they know your location (such as online mapping, or searching for specific things in your area). When you visit one of these sites, Firefox will ask you if you want it to provide the site with your current location.

If you wish to allow Geolocation, Firefox first check for relevant location markers for instance the location data provided by a GPS device built into or attached to your computer or browsing device, the signal strength of nearby wifi system and/or cell phone towers.