Initially, sites guess your location from your public IP address. This typically is assigned to your current internet service (home, office, mobile, hotel, coffee shop, etc.). Your IP address could be more distant if you use a proxy server or VPN or other technology that changes how you connect to websites.

If you give a site permission to access your precise location -- the prompt looks like this:

-- then Firefox will ask Windows for information on nearby WiFi hotspots and send that to a Google location service to get your estimated lat/lon coordinates and then send those to the site. Usually that is pretty accurate.

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