I am using Thunderbird 31 on Ubuntu. I set up my accounts and then I closed Thunderbird.
But I'm not getting any notification.
Is there any add-on I need?
Thunderbird needs to be open if you want to get notifications.
There is an addon called
Thunderbird needs to be launched to be able to see new message notifications.
It can be minimized, of course, and the email icon at the top of your screen will turn blue in colour when new mail arrives and the icon in the launcher will indicate the number of emails yet to be read.
You can also enable Workspace Switcher in System Settings > Appearance> Behaviour (if not already done so) and launch Thunderbird in another workspace so that it doesn't detract from screens you use for launching other things.