Rated 5 out of 5 stars

Absolutely essential add-on. However, one minor suggestion:
When triggering the dialog, it would be helpful to have the entire subject line already selected so one may simply begin typing to replace it (rather than first pressing Ctrl-A/Cmd-A to select all the text). Admittedly, a minor quibble.

This review is for a previous version of the add-on (4.2). 

Rated 5 out of 5 stars

Absolutely valuable add-on. I've found one bug but it's easy to work around.

If you have two eMails with the same subject (like a report of charges on your credit card), changing the subject of the first works fine but, due to the way the add-on works (apparently duplicating the eMail with the now-new subject and then deleting the original eMail), attempting to change the subject of the next eMail (with the same original subject line) actually deletes the wrong eMail. I'm certain this is just a "timing" issue as, if I wait until the original eMail disappears from the list before attempting to change the subject of the next eMail (with the same original subject), the problem doesn't occur. Now that I know what (not) to do, it's okay.

Perhaps the developer will look into this. Having said that, I wouldn't be without this add-on. Thanks.

Edit: using add-on for TB78. (Not sure how to reply to your ? except by editing my review.)

This review is for a previous version of the add-on (4.2). 

Which add-on version are you using? The one for TB68 or TB78?

Rated 4 out of 5 stars

Somewhat helpful add-on; however, as noted, it's writing to the local TBird database and doesn't really -change- the subject line of the eMail. It's this latter feature that would be much more useful as, once I move to another machine, whatever changes I made on PC#1 don't appear on PC#2. As well, when I had some trouble (not related to this add-on) that required I trash my TBird profile and start over, all the changes I made were gone. (TBird 73.x)

{edit:080220} I understand wiping out the profile and re-generating will lose the edits to the local database. I just located the "Options" checkbox that, if unchecked, says it will write the changed Subject line directly to the eMail (which is what I would prefer so changes appear on all devices accessing the eMail). But is there any danger in doing this? I only use IMAP mail. Thanks for any assistance you might provide.

This review is for a previous version of the add-on (4.2). 

Glad you found the option. There is no danger in using it together with IMAP.