Jonathan,
I'll try rebooting. I thought restarting the program would make the
change, but perhaps not.
In my experience neither was necessary - the option took effect as soon as I closed the config editor and Options dialog.
You do need to be running a new enough version of Thunderbird: 52.4 or later. But you shouldn't even see the option if you were running an older version.
In that version and later the default value of "mail.override_list_reply_to" is "True", you want it to be "False" to restore Reply-To behavior.
Shal
Shal Farley