Firefox 3.5 has shipped with location finding turned on: The latest release of Firefox includes by default the option to use a computer's IP address and, if available, a scan of nearby wireless networks to provide a location to Web sites that use appropriate JavaScript to request a position. Users can opt out when asked, disable location requests for a site, or disable location requests entirely. However, "ask for permission" is on by default.
Firefox is using Google Location Services, which is a combination of cellular tower data that the company has assembled along with some unknown method of collecting and locating Wi-Fi hotspots, much as Skyhook Wireless has been doing for years. Likely, Google gathers this information as it drives the streets for Google Maps.