Firefox uses its own notification system for alerting users to various events. These can easily be missed on an Ubuntu system as users are used to seeing notifications appear in the top right-hand corner.
Install
All you have to do is add the following extension and restart. Firefox will, from now on, use the system notification system.
https://addons.mozilla.org/en-US/firefox/addon/9622
Quite why this extension isn’t installed by default in Ubuntu i don’t know!