Rated 5 out of 5 stars

I love this extension, but the TB 3 problem with multiple addressees bothers me, too. One way I could envision overcoming TB 3s problem would be to set this extension to 2 line mode, and set TB 3 to View -> Headers -> All.

Then, when there are too many addressees to view, ignore TB 3’s “more” button and just press the “plus sign” to expand the header to a reasonable size and then have this extension modified to automatically press the “more” button, if one existed, with-in the expanded header.

That way a single press of the “plus sign” would reveal a very large header area with, most likely, all the addressees listed. If not there would be the scroll bars to continue.

The only modification of this extension would be to automatically press the “more button”, if one exists, after the user presses the “plus sign”.

I don’t know why there is a “more” button inside of the “all headers” feature anyway. All headers should be all headers!

Thanks for listening and thanks for the extension!

This review is for a previous version of the add-on (1.1.4). 

I agree with you that it would be a good idea to show all email addresses of the To/CC fields in the full header mode.
See https://bugzilla.mozilla.org/show_bug.cgi?id=536542