The "Edit HTML" appears to be outdated. In Thunderbird, it can be found
but with a statement that it is not available for Thunderbird 16.0.1.
And there are user comments like the following:
Certainly an odd statement. I have it installed since way back, worked
in 16.0 and also now after updating to 16.0.1 .
"It sounds like it once was a good extension [...] However, this has not
been updated since 2007."
In my opinion it works great as is and needs no updating. Not like some
which have developers playing around seemingly on a whim with UI, etc.
The author is/was a prolific extension author and was/is very receptive
to requests for bugs, etc. which were responded to in short order.
But information about "Edit HTML" also mentions the "Stationery" add-on,
which has a somewhat confusing description (probably because it has some
extra features too), but it seems to be very simple. It changes e-mail
message composition window so that it has two tabs, normal (wysiwyg) and
HTML.
I recall "Stationery" only in that the author of this add-on interjected
at times during discussions re. EditHTML with requests to try his. I'm
certain that "Stationery" is a fine add-on, however we stayed with
EditHTML since it did what we wanted it to do.
I'm very suspicious of any statements/comments from Mozilla people since
the majority are contra-HTML-mail. Most know nothing about HTML Mail and
couldn't care less. A group of us advised users interested in a group
netscape.test.multimedia which changed to mozilla.test.multimedia but
those contra-HTML forces destroyed the group and is now mostly dead. It
did not surprise me when Mozilla again pushed TB aside. It does not
surprise me that HTML is totally ignored but that social messaging is
embraced. The instruction for toggle of using shift+write etc. to put TB
in HTML mode is hard to find. I tried unsuccessfully to find a way to
set it to HTML in the Menu Bar. Seems to be a well kept secret and is
intended to remain so.