Generally I write email using plain text. Sometimes I really want to use HTML. I couldn’t find an easy way to switch to compose in HTML when composing a message in Thunderbird so I started searching for extensions. I found that you don’t need an extension to accomplish this. All you need to do is hold shift while clicking on the compose button (“write” button). This doesn’t work with the keyboard (ctrl-shift-m) but I’m OK with that since it’s rare that I compose in HTML.
I also found that holding shift while clicking on Reply or Reply to All also opens the composition window in HTML editing mode.