I believe you need to have the explicit id as shown in the TO list also defined in your addressbook. There is also a config editor entry, mailnews.reply_to_self_check_all_ident , but I'm not sure if it accomplishes your goal. As it stands, 'reply all' is doing as it states.

Thanks David, that has helped me to work out what is happening. The email address I use is a @gmail address, but when I started using it 7 or 8 years ago it was a @googlemail address. As far as Google is concerned they are the same. It seems that the @googlemail version is still in some peoples' address books. When somebody emails me using the @googlemail address, the emails end up in the same inbox in Thunderbird. When I "reply all" the "from" defaults to my @gmail address, but it seems Thunderbird treats the @googlemail address as a different address, and includes it in the recipients. If I "reply all" to an email that was addressed to my @gmail address, it doesn't send me in the recipients. I've added the @googlemail address to the address book entry for my @gmail address, but that doesn't seem to have fixed the problem. If I go to Google to do my mail, or do it on my Android phone, a "reply all" doesn't include my address in the CC list, presumably because Google recognises that @gmail and @googlemail are effectively the same address.

I agree with what you're saying, but TB doesn't act as Google and Android do. To my knowledge, this behaviour is also present in Outlook. When you next receive a message to a group and you reply, I believe you will be okay if address entries match. But 'reply all' is still a possibility.