New version of Mailbox Alert!
There have been a lot of changes in the internals, it is queue-based now, you only get one alert when there are multiple new messages in a folder. You can also specify complete accounts, and some bugs have been fixed.
See the complete Changelog for more information. Or, if you’re in a hurry, you can download it directly.
Since Mailbox Alert is on addons.mozilla.org, the automatic update will from now on check there, and not my site.