I cannot duplicate your problem. When I click "Reply All": the Original Sender - as well as ALL of the CC: And TO: recipients on the message - are addressed properly. Addresses that were sent BCC: to shouldn't show up in the "Reply To" addresses ...

Run TB in safe mode (hold Shift when you launch TB). Does that change anything?

@Sea Mac: Well, it would depend on the specific e-mail, so testing it on one or two will not duplicate the problem. Reply all to Google Groups and Mailman does include the From: sender. Reply all to old Yahoogroups mails from prior to the DMARC switch also includes the From: sender (note: Yahoogroups mails from *after* the switch rewrites mails so that the From: sender's address is the same as the From: sender's address, so that's no help). Reply all with Groups.io does not add the From: sender (note: Groups.io sometimes does the same as Yahoogroups, depending on the domain, but I'm testing this on mails that contain the actual sender's address in the From: header).

@sfhowes: Running safe mode does not alter the behaviour.

I guess what I'm asking is: is there a way to make it so that when I click a reply-like button on the e-mail viewing pane/window, the e-mail address from the From: line in the header gets added as a recipient (or one of the recipients) in the e-mail that I'm about to write?

I faintly recall there were some changes made last year about replying to mailing lists, but I'll have to look for it and reply later (no pun intended).

I think the preference mail.override_list_reply_to was added to restore the old behavior.