Yettwasezmel 3 ɣef 5 n yetran

sɛur JohannesK di Jan. 3, 2015 · aseɣwen yezgan

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.