Review for FolderFlags by Darkhoof
Rated 5 out of 5 stars
This add-on allowed me to set the "Deleted Items" folder of my institution e-mail provided by Microsoft an fowarded by IMAP as the Trash folder in Thunderbird.
There was no option in Thunderbird to define the Trash folder. Weird, considering that you can pretty much define any other folder.
This was done in the latest stable release so 5 out of 5.
To create your own collections, you must have a Mozilla Add-ons account.