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Just what I wanted !

Talking about threads, do you know something which could join two threads, or split one thread in two ? (sometimes thunderbird don't sort really how I would want...)

Это отзыв для предыдущей версии этого дополнения (0.4). 

There are some user prefs to change threading behavior and there is a longstanding bug for customizing threads (adding messages to a thread or splitting a thread). See bugzilla:

https://bugzilla.mozilla.org/show_bug.cgi?id=36024