For TB 91.3.1 and earlier, the add-on allows to select a charset for the message. With preference `extensions.CharsetMenu.fixThreadTree` set to true, it will also fix the subject in the thread tree.
From TB 91.3.2 and from TB 94 beta, the API nsIMessenger.setDocumentCharset() to set the charset of a message has been removed, relying fully on auto-detection. For these versions, the add-on only shows the detected charset.
The add-on addresses the following issues:
The charset menu isn't working for Japanese charsets already in TB 78
and furthermore the charset menu has been removed in TB 91
and the new repair option has only been fixed in TB 91.3.2 and from TB 94 beta
Note that from 91.3.2 and TB 94 beta the repair option will not repair the subject in the header pane as setting a specific charset used to do in TB 91.3.1 and earlier. This is tracked here:
This bug also covers the issue that the detected charset is not always correctly reflected in the UI. Please contact the author for a workaround.