I use plain text mails. Very rarely do I send out HTML mails. Those times are a pain for me as I usually reconfigure the MUA and restart it in order to send HTML mails (can't exactly recollect when I last did it).
Today, I found this neat tip (thanks to the world of Blogs) - Press "Shift" key while clicking "Write" button on Thunderbird. That opens up an HTML edit window.
For more such tips, go here. And, visit knowledge base for Mozilla projects.