Thunderbird doesn't have any native support for modifying header info such as the subject line. There are add-ons that allow this, but they appear to work by altering the summary (not the original message) and so the modifications are lost if you re-index. I've never seen it stated explicitly, but I suspect Thunderbird has a policy of not tampering with message content in any way. Unlike some email clients, it stores the message as received, verbatim, and doesn't separate out the headers or any attachments. I suspect that in most cases you could, if you were so inclined, open the relevant mailbox file and edit the subject line there, using your favourite text editor. And then you'd probably have to re-index to get the summary to reflect the change to the message.