Rated 5 out of 5 stars

Update: Five Stars!

Many thanks to John Bieling for pointing me to the Help resource that explains how to configure add-on options. Following those instructions, it was easy to change behaviour to IMAP-compatible. A couple tests showed Subject changes were quickly reflected in the server and visible when viewed from my cell phone or web browser (Firefox, of course!).

Maybe it would help others in the future if "About this Add-on" included the link to "How do I configure add-ons". Just sayin'...

Thanks again for your help. Sent donation a few minutes ago!

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Does not work as advertised.

"About this Add-On" says:
Since 2.0 version in the options:
1- Changes are done into local TB messages database, not into the message (Subject from Header doesn't change).
2- Changes are stored directly into header, so it is IMAP compatible.

But I can't find any option to choose behaviour #1 (local DB) or behaviour #2 (IMAP compatible). Add-on seems to provide only behaviour #1 and no option to switch to IMAP-compatible option.

I guess I'll go back to Outlook until this function works well in Thunderbird.

(TB 78.10.0 (64-bit), Win10)

Have you read the Thunderbird help on how to set add-on options? You should do that. It actually has THIS add-on as an example.

https://support.mozilla.org/en-US/kb/thunderbird-add-ons-frequently-asked-questions#w_how-do-i-configure-add-ons