It might be nearly useless, it might even be a waste of time, but this is my favourite Mozilla Thunderbird add-on.
It displays a fancy icon which immediately tell which e-mail client the sender of the message is using. As of today, it has got icons for 564 different softwares/webmails/... It's so fun to browse e-mails to discover who's using the strangest e-mail client. :-)
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