Comment 282 for bug 52667

(From update of attachment 380027)
in general, looks good.

Are you using these?

diff --git a/mailnews/base/public/nsIMsgDBView.idl b/mailnews/base/public/nsIMsgDBView.idl
--- a/mailnews/base/public/nsIMsgDBView.idl
+++ b/mailnews/base/public/nsIMsgDBView.idl
@@ -206,6 +206,9 @@
   const nsMsgViewCommandTypeValue applyFilters = 30;
   const nsMsgViewCommandTypeValue runJunkControls = 31;
   const nsMsgViewCommandTypeValue deleteJunk = 32;
+
+ const nsMsgViewCommandTypeValue replyToAll = 33;
+ const nsMsgViewCommandTypeValue replyToList = 34;
 };

Unless I'm missing a later use of msgURI, I think this:

+ let msgURI = GetLoadedMessage();
+ let msgHdr = messenger.msgHdrFromURI(msgURI);

can just be
+ let msgHdr = messenger.msgHdrFromURI(GetLoadedMessage());