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This addon adds an entry in the right-click menu which re-threads selected messages together (groups selected messages into the same thread)". This is done by _modifying_ messages; the author of the addon declines any responsibility for data loss.
About this Add-on
References:header. The modified messages are injected into their respective folders, and once this is successfully done, the old messages are deleted. Thunderbird then figures out that all the messages belong to the same thread since they have a common ancestor.
There is no way to undo the operation and I don't plan any further improvements to the addon. The addon only deletes messages after the modified versions have been injected in the folders. Yet, there is still a chance the modification process may damage your message. You have been warned.