Within Thunderbird, I compose all my email in plain-text and view messages with remote content blocked. Sometimes, though, I do have the occasional need to forward an HTML email from a trusted source. For as long as I can recall, I've been rifling through the main Settings menus, setting the
Compose in HTML switch, forwarding the note, and then clearing the switch. It's a pain.
After years of Thunderbird use I finally learned that you can
shift+click the Reply or Forward (or Reply-All) buttons and it will, for that message only, toggle HTML composition.
It's the small things in life.