I just got my very first self-made Firefox add-on approved on AMO (https://addons.mozilla.org). It’s a simple add-on that shows a notification in the operating system when a YouTube video starts playing. Maybe a bit pointless if you are watching videos, but it may be useful if you are using YouTube to listen to music in the background. It’s called YouTube – Now Playing Notifications and can be found here:
As I said, this is my very first add-on, and I must say that I really like Mozilla’s developer tools. Developing an add-on with their add-on SDK is really easy and it’s also really easy to upload add-ons to AMO.