From MozillaZine Knowledge Base
A folder URI is an internal identifier for a folder in a Mozilla mail application like Thunderbird.
You sometimes need a folder URI to identify a folder when you are configuring your mail in advanced ways. This article explains how to specify the URI for any folder. The form of the folder URI depends on the type of account that contains the folder.
Special characters in URIs are encoded in the normal way for URIs. For example, a space is encoded as %20 and an at-sign (@) is encoded as %40.
This often affects URIs where the username is an e-mail address. The at-sign in the e-mail address must be encoded, but the at-sign that is part of the URI must not be encoded.
You can use some extensions to find folder URIs, avoiding any problems with encoded characters.
You can use the keyconfig extension to set up a shortcut key to display the currently selected folder's URI. Program the key with code like:
with (GetSelectedFolderResource()) prompt("URI: " + URI, URI)