Rated 5 out of 5 stars
The addons "FiltaQuilla" and "Shedule Filters" should be built-in features of Thunderbird.
for (let index = 0; index < msgHdrs.length; index++)
let hdr = msgHdrs.queryElementAt(index, Ci.nsIMsgDBHdr);
hdr.subject = "[Hello, world] " + hdr.subject;
It displays the [Hello, world] in the message list, but its not saved to the IMAP server and after moving the mail to another folder this change disappears. :(