When you visit a location-aware website, Firefox will ask you if you want to share your location.
If you consent, Firefox gathers information about nearby wireless access points and your computer’s IP address. Then Firefox sends this information to the default geolocation service provider, Google Location Services, to get an estimate of your location. That location estimate is then shared with the requesting website.
Mozilla: Location-Aware Browsing
via The Map Room
I wonder how it finds out about nearby WAPs? Is Firefox 3.5 effectively a wifi scanner now?
The Google Code Blog gives more background
With recent launches like Google Latitude and Google Toolbar with My Location, it's clear we think location-based services are useful and fun! We also want developers everywhere to be able to use location to build cool new features and applications.
I'm pleased to announce that Google's Location Service (the same service which powers the My Location feature in many Google products), is now the default location provider in Mozilla Firefox 3.5 beta 4. This means that developers can, with users' permission, gain access to their approximate location without requiring any additional plug-ins or setting configurations. It just works!
Mozilla Firefox 3.5 beta 4 uses the W3C geolocation API, a standard being adopted by many major browsers, so you won't have to branch code in the future.
Google Location Services now in Mozilla Firefox - April 30, 2009
In a quick Google search, I didn't find any page specifically on "Google Location Service" itself.