A click on the Share Location button will share the current location with the website while a click on Don’t Share will prevent it.

To be precise, the following information will be send to the website:

Your computer’s IP address,
Information about the nearby wireless access points, and
A random client identifier, which is assigned by Google, that expires every 2 weeks.

Firefox users who never intend to use the geolocation feature might want to disable it permanently. This can be done easily by typing [about:config] in the Firefox address bar to open the Firefox configuration. The feature can now be disabled by entering [geo.enabled] into the Filter form at the top and pressing the enter key. The geo.enabled parameter should now be displayed. The easiest way to disable geolocation is to double-click the entry so that the Value displays false afterwards.

You find more information about geolocation on the Location-Aware browsing page over at Mozilla which also explains how to get remove permissions that have already been granted to websites and the random client identification number.