Available updates...

[timgood]

The update icon in system tray is not showing any updates available. However, running 'sudo apt-get update' and then 'sudo apt-get upgrade' results in both Firefox and Thunderbird being upgraded. Why does the system tray update icon not show available updates?

[Inkit]

I don't know the exact reason why this is so, but it is a known problem and there is even a post by Clem from years back detailing why this is so. I have however found that if you change the default from update every 15 min to once every few hours, you actually get notified whenever you have an update. You can change this in preferences in the update manager.