Review for Mailbox Alert by Die Mensch-Maschine
Rated 5 out of 5 stars
Does what it says it does. The current thunderbird alert system is useless for anyone who receives a lot of mail. This extension is an excellent replacement.
A feature I would like to see in the future would be the ability to get alerts on receipt of mails from a specified list of addresses, or to specify a list of addresses you don't want to trigger a notification.
To create your own collections, you must have a Mozilla Add-ons account.