Rated 5 out of 5 stars

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.