Thunderbird Tip

From this excellent collection of Thunderbird tweaks, I found exactly what I was looking for (described as I would have described it, had I known it existed and I was writing it up):

Like any responsible internet user, you have your default email format set to plain text. However for those rare occasions that HTML becomes necessary, you can easily toggle to it by Shift-clicking the toolbar buttons. So Shift-Compose will compose an HTML message, etc. (At time of writing, this inexplicably does not work for forward.)