Review for MinimizeToTray revived by Loren Dias
Rated 5 out of 5 stars
Works on Gnome 3.16 + Arch Linux + Thunderbird 38 Beta.
Doesn't have the fancy Mailbox icon that FireTray has, however it seems to be simple and effective, and unlike FireTray it's set to "double-click" to restore - which is more consistent with other Tray icons.
To create your own collections, you must have a Mozilla Add-ons account.