This is caused by a feature that is part of Firefox that checks for updates of your currently installed extensions. If any of your extensions have specified an update URL, Firefox will periodically check that URL for updates.
You can turn off this update checking feature in Firefox by going to the "Options" window, then click "Advanced", then "Software Update", and then uncheck the "Periodically check for updates to: My Extensions and Themes".
This means that you are using an old version of Firefox which does not understand the new installer. This new installer was introduced in Firefox 0.9 and does not exist in versions prior to that.