MarkSubfolders New is a Thunderbird extension that enables users to mark emails in a folder and all of its subfolders as read.
MarkSubfolders New is an open source add-on designed to work with Mozilla Thunderbird 1.5 and later.
How to Install in Thunderbird:
1. Right-click the link below and choose "Save Link As..." to download and save the file to your hard disk.
2. In Mozilla Thunderbird, open Add-ons from the Tools menu.
3. Click the Install button, and locate/select the file you downloaded and click "OK".
Product's homepage
Requirements:
· Mozilla Thunderbird