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.
Thankfully the following Firefox add-on solves this by getting Firefox to use the Ubuntu notification system. No-more double-downloading because you missed the pop-up!
All you have to do is add the following extension and restart. Firefox will, from now on, use the system notification system.
Quite why this extension isn’t installed by default in Ubuntu i don’t know!