In Thunderbird, while composing an email, how to convert it from plain text to HTML?

In Thunderbird, while composing an email, how to convert it from plain text to HTML? In general, I'd like to compose plain text emails by default. But sometimes, when I realized I need to insert HTML-supported elements in the email currently being composed, how do I convert it to HTML? If possible, after the current email is completed, I still want plain text email composition by default.

I agree with the comment above that it's not possible. However, it's possible to workaround.

Upon composing a plain text email and realizing that it should be in HTML, you can do the following (which I tested in TB on Win10)...

1. Either hold down Shift and click the Write button on the main TB window OR click Message in the New menu on the Plain Text message window while holding down Shift. A new message window including a formatting toolbar should appear, where the presence of the formatting toolbar implies the format is HTML.
2. In the Plain Text window, type Ctrl+A to select the entire message.
3. Drag & drop the selection from the Plain Text window to the HTML window.
4. Close the Plain Text window.
5. Continue composing the message in the HTML window.

Of course in step 3, you can use any other copy & paste method if you prefer.

The simplest method, in my view, is:

1. Ctrl+s or menu-item File: Save to save a draft copy of the current email under composition.
2. Right-click on that saved draft in the Drafts folder and shift-click on the Edit as new message option to bring up an HTML version of the email to continue editing.
3. Discard the original email pane.