Thunderbird 3.0 and higher may display a new "reply list" button instead of "reply all", when the message appears to be part of a mailing list thread. "reply list" deletes all other To: and Cc: recipients, and it is the default action whenever it is present.
There are at least two situations in which this behavior is undesirable:
1) When one or more thread participants in the To: or Cc: lists are not actually subscribed to the mailing list themselves. This is common when dealing with high-volume lists.
2) When thread participants have configured their mail readers to flag messages in which the user appears in the To: or Cc: lists (indicating that those posts deserve more attention than the general "background noise" on the list). For instance, Gmail displays an ">" or ">>" icon next to threads in which the user has received replies, and displays nothing otherwise.
This add-on simply overrides the check for IsReplyListEnabled(), unconditionally returning false. Thunderbird will then fall back to displaying the "reply all" button, as it did in previous releases.