So, here is the initial problem, Mozilla has purged Growl from their code base entirely, all new versions of Firefox and Thunderbird will no longer talk to any version of Growl directly. They would not work with us to resolve the issues they felt they had, which is unfortunate, but luckily there are alternatives. Plugins like the one you linked can add the functionality back, however there are often things you have to configure, from their own documentation:
Some Thunderbird preferences which have something to do with the behaviour of the extension:
- Tools -> Account Settings... -> Server Settings -> Check for new messages ...
- For IMAP folders: Properties... -> General Information -> Check this folder for new messages
- For POP3 accounts: Tools -> Account Settings... -> Server Settings -> Automatically download new messages has to be checked
- The nature (inbox, bulk ...) of folders
- Enable mail.check_all_imap_folders_for_new for notification on all folders by default (useful when using filters)
It's very unlikely that Growl is preventing notifications from the other folders, and it has to do with settings for that account/folder affecting how the plugin functions. It is possible that Thunderbird has changed other behavior and the plugin needs updating or fixing, which since we didn't create it, we can't really help with.