Review for NTFNTF by Wilson
Rated 5 out of 5 stars
Perfectly does what it advertizes. It doesn't claim to have custom sounds, but there are even a set to choose from (at least on MacOS, I can't speak for other OS). I don't understand the bad reviews about this feature, that the add-on doesn't claim to have. It doesn't provide world peace either, is that a reason to give a bad review?
The documentation for setup is great, even for MacOS, where the notifications are done by the OS. Perfect if you use sieve filters and don't want to get notifications for new mails in folders but only for the inbox. You can choose a different sound for each account e.g. to differentiate between work and private mails.
Highly recommended add-on!
To create your own collections, you must have a Mozilla Add-ons account.