Sets the urgent flag (X11) of the Thunderbird window under Linux in case of new messages. Does ignore special folders like Drafts, Junk and such.
Version 5.0 uses MailExtensions API. Since onNewMailReceived event had been implemented firstly in Thunderbird 75, this is the minimum Thunderbird version it supports.
Version 4.0 uses WebExtension Experiment API which is subject to change.
Version 3.1 uses the legacy API, it is guaranteed to work up to version 78.
The original addon: https://addons.thunderbird.net/en-US/thunderbird/addon/seturgent-new/
The .xpi files are on the releases page.