This addon checks whether HTTPS sites you visit are vulnerable to the Heartbleed attack, and if so overlays the bleeding heart logo over the contents.
The add-on checks the site's vulnerability by asking a Mozilla-hosted server. Responses are cached so the number of visits are obscured, but a record of which HTTPS sites you visit are sent to that cloud server. Being Mozilla we have no interest in storing that data and don't, but if you're concerned you can disable the add-on after you've checked out your usual sites.
Since this is a diagnostic tool I've left debug messages on. You can see exactly what the add-on is sending to the server on the Error Console.