Note (November 17th, 2017): For users on Firefox versions less than 57, if you have a version >0.2 please manually downgrade to version 0.1.3b. An update was available for a few days that did not have the correct minimum Firefox version number set, and it is not possible to fix this automatically due to Firefox restrictions.
This add-on scans JPEG images automatically as the browser downloads them to detect when they contain embedded GPS coordinates in their EXIF information. When it finds such an image, it shows an alert and links to a map. It also maintains a history list accessible from its toolbar button.
A useful test for verifying that this works can be done by loading the full size versions of any of the images on this page: https://commons.wikimedia.org/wiki/Category:Media_with_GPS_EXIF