Firefox 3.1 supports the [geolocation API specification](http://dev.w3.org/geo/api/spec-source.html).
Try copying this code and running it in Firebug:
```javascript
function showMap(position) {
// Show a map centered at (position.coords.latitude, position.coords.longitude).
console.dir(position);
}
navigator.geolocation.getCurrentPosition(showMap);
```