It took a while, but my first patch to Thunderbird was committed today!
changeset: 2727:98a7de404c08 user: Blake Winton <email@example.com> date: Fri May 29 10:35:37 2009 +0100 summary: Bug 45715 - ""Reply to List" [button/(context) menu item]" [r=mkmelin,sr=bienvenu,ui-review=clarkbw]
The patch started off by adding a “Reply to List” button to the message header pane as seen below, but after some discussion, the scope was expanded to change the “Reply” button to “Reply All” or “Reply to List”, depending on the message you’re currently viewing.
Of course, there’s still some things I’ve got to add, but those can go in a separate patch, which will be much smaller, and so much easier to get reviewed and committed. And once it is, we might be able to close a 9-year old bug, which would be pretty sweet.