Is it possible to combine "Reply To All" and "Reply To List" buttons in Thunderbird's toolbar? I'd like it to act as "Reply To List" if possible, and as "Reply To All" if I'm not replying to a message from a mailing list.
What you're looking after, does already exist: The "Smart Reply" button.
It's not in the standard toolbar, but you can add it by customizing the message view toolbar (through right-click on the message view toolbar) and adding the Smart Reply button - it does precisely what you want :)
Note: This button only exists in the message view toolbar, not in the main toolbar:
I have a Smart Reply button over here:
Which I can drag and drop to the toolbar, which I did here (and additionally removed the normal Reply button):