ToggleReplied 2.0.1
by Jonathan Kamens
Enables toggling of replied and forwarded states of messages.
About this Add-on
This add-on adds a second "Mark" menu near the bottom of the context menu for the message list. It's a separate menu because Thunderbird does not currently allow add-ons to add menu items to submenus created by Thunderbird itself.
On this submenu are two items, "As Replied" and "As Forwarded". If the message you right-clicked on to bring up the menu was already marked replied and/or forwarded, one or both of the menu items will be checked. Selecting one of the menu items will either mark or unmark the message as appropriate, i.e., it will reverse whatever the state of the checkbox is.
Beware that if you select more than one message before clicking, the add-on looks at only the first of them (where "first" is defined arbitrarily by Thunderbird) to figure out whether to mark or unmark messages.