Comment 25 for bug 10883

(In reply to comment #22)
> Sometimes I need to compose in html and there is no way to it.

There is, actually -- holding down shift while clicking the Write or Reply
buttons will toggle the compose mode for the new window -- if you normally
compose plain, it will open an HTML mail window, and vice versa. (Also works
for the Forward button if you are setup to forward as attachment; forward inline
causes the shift to be ignored.) No equivalent exists for Edit As New.

But that's just a workaround. It would of course be more flexible if there were
a way to toggle the compose mode once the compose window has opened. One
drawback is that toggling from HTML mode to plain mode will lose any formatting
(per Scott's comment 2), and if you toggle accidentally, toggling back will give
you just the basic conversion without restoring the formatting. I suppose it
could be made Undo-able.