It is here! Mailbox Alert 0.16.1!
0.16 should already fix most (if not all) the ongoing problems with the cursed 0.15 release.
But as my previous post already said, not all woes were over, and I made a mistake when uploading the final release file. This update should correct those.
See the full changelog here:
And get the addon itself if you are not using addons.mozilla.org here: