The difficulty is that this answers the wrong problem. The real problem is that some users send you mail in answer to a thread but it doesn't have the Message-ID in the References, so despite the Re: in the Subject line, it appears as a new thread. What is needed is a way to force such messages back into the *existing* thread that they ought to belong to, *preserving* the OP's original Message-ID. This add-on fails on both counts: it creates a new thread instead of preserving the original one, and it trashes the original Message-ID so that any more messages arriving won't have a thread to attach to.
Doesn't work for me. Selecting a bunch of messages to be rethreaded that some clown had thread-hijacked and then choosing "Rethread" only causes every message to become an immediate reply to the "root" message of the original thread regardless of the cascaded level. i.e., This:
I haven't tried this extension yet, at the moment I'm using Header Tools Lite https://addons.mozilla.org/thunderbird/addon/header-tools-lite/ to set the References-header manually. In the description of that extension I found this: Some IMAP servers, such as Gmail, do not accept changes if some basic headers (date, sender, the message id, object) are not modified . With the option “Forcing changes with IMAP accounts enabled“, the changes are forced by changing the date of the message of 1 second.