https://www.gmass.co/blog/use-custom-html-in-gmail-compose-window/
Option 2:
right-click anywhere in the Compose box and choose Inspect
This is Gmail's mail compose box. This leads to the HTML code of the web page, pointing to the div element of the mail under composition.
There with right-click on the element a menu comes up whose first entry is
Edit as HTML
where you can insert your pre-written HTML code.
And this worked for me with Firefox.
