Update Notifier by Manuel Reimer

This Add-on helps you to have your browser as up to date as possible as early as possible. On startup the Mozilla server is asked for the latest version and if this does not match what you are currently using, a warning message is displayed.

This is especially useful if you manually update Firefox using, for example, your Linux distributors package repositories. But even with the Firefox built in update mechanism, the update is prepared in the background and applied on the next browser restart which results in one full browser session with potential security holes.