Thunderbird first letter folder nav add-on

Don H

Running Win 10 and Thunderbird version 91.11.0.
I had the thunderbird addon for first letter folder navigation installed and running well but for some unknown reason it simply dissapeared. I forgot the actual name of the thunderbird addon. Can someone please tell me the name of the addon. I am not talking about a NVDA addon but a thunderbird addon.

Join { to automatically receive all group messages.