I normally send e-mail as plain text.  It’s smaller, satisfies all of my formatting needs when composing messages, and avoids dancing crap, goofy fonts and eye watering colors.  However, once in a while I have to forward something that uses HTML formatting or reply to something that already contains formatting that I don’t want to mess up.  I’ve been using Thunderbird as my mail client for five or six years, but could never find a way to do it.  Today I did.  This blog post points out that holding the shift key while clicking the reply button will do exactly what I want.  This works in Thunderbird 3; I don’t know about earlier versions.  Note that you can’t use it with Ctrl-R, as that will reply to all, rather than reply in HTML.  Grazie to the author (whose name I couldn’t find).