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Great - but it seems that I have to choose this every time I view the message. Is there any way to set this persistent, and for a whole folder, not a single message?

Questa recensione riguarda una versione precedente (1.1.1) del componente aggiuntivo. 

No, the setting is forgotten when you switch between messages. The Thunderbird team decided to remove the "per folder" setting, see: https://bugzilla.mozilla.org/show_bug.cgi?id=1671880 (the folder charset was removed in part 2 there). For news, the server default charset is used. If you want to discuss the matter further, contact me via e-mail.