That would depend on the browser and its settings. It might use GPS, WLAN AP-based location, IP location, or any other conceivable method of locating itself.
The browser runs on your local machine, so even if you're behind a proxy, it should be able to find your actual IP address (unless security features prevent it from being sent for lookup to a geolocation server). Whether any browser is smart enough to handle being behind a NAT (with the local machine having a private IP) and looking up the gateway's IP address, that I don't know.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
