Send a single HTML email from Thunderbird, overriding the default "plain text" setting

If your default setting is to compose Plain Text emails, hold down the Shift key when you click the Write button or Reply/Reply-All/Forward button. This will open a window to compose an HTML email.

Another possible solution is to select the box

Account Settings -> Composition & Addressing -> "Compose message in HTML format"

and also select

Tools -> Options -> Send Options -> "Send messages as plain text if possible"

Then when you open a composition window, you'll see the HTML formatting bar (font size, bold, italics, etc.), but Thunderbird will only send your message as HTML if you use one of those options (or if you're replying to an HTML-formatted message). If you compose a plain-text message with no formatting, Thunderbird will detect that and send the message as plain text.