I just found the new extension (as of Sept. 2011) editemailsubject
(must be typed as written, without blanks, in Thunderbird's extensions
search). It allows to edit the subject displayed in the mailbox window
while preserving the subject in the message header. Exactly what I was
looking for.
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The TB Header Tools were not updated for a long time - the latest release on
addons.mozilla.org is for TB 0.5-1.0 and the 7.1 update for TB 3.0 is just an
unofficial hack. So it seems TB Header Tools will never be updated to TB 5.
I sometimes get messages without subject (from people in a hurry)
