Worked for me.. TB 52 upgrade started replying to individuals instead of the google groups mailing list I was on.
Installed easily by downloading the .xpi file (whatever that is, I guess it' s like a zip or tar), and using thunderbird -> Tools -> Add Ons -> Extensions -> ("gear" pulldown menu) -> Install add on from file, restart, and now "Reply" works.
Yes, finally the broken behavior of Thunderbird if fixed. I had so many maillist messages sent in error to just the sender instead of the whole list, or even if I remembered, it was cumbersome to copy & paste the maillist address.
Thank you for this addon.
This review is for a previous version of the add-on (1.1).