Rated 5 out of 5 stars

While editing, can't copy or cut anything. Paste works as expected.

Update: C-E responded quickly with a work-around. Tbird is the culprit. Thanks.

This review is for a previous version of the add-on (1.0.2). 

Sorry, this is a bug in Thunderbird, see
https://bugzilla.mozilla.org/show_bug.cgi?id=1692327
I cannot do anything here.

Only the keyboard command Ctrl+X for cut seems to work.
As a workaround for copy, you can do Ctrl+X followed by Ctrl+V.

Rated 5 out of 5 stars

Works great. HTML code is great. One suggestion:
Attach a name/label instead of a number. HTML can get quite extensive and trying to remember what each does (by number) is a little challenging.
I use a cheat sheet now, and that works fine.

This review is for a previous version of the add-on (1.0.2).