Rated 3 out of 5 stars

I don't see this working, I have the config set up but there is no button or option.
Edit: by turning on the Composition Toolbar in the compose window I can see the button, but it's grayed out and has no effect. I still can't find the shortcut, what is STRG?
Tbird Windows 78.4.3

This review is for a previous version of the add-on (1.3). 

Note that the default shortcut was incorrectly advertised here as ALT-W, it's STRG-SHIFT-W again. Might clash with other addons, though.

The button can be added when opening the customization dialog in the compose window (right-clock on button bar).

Otherwise drop me an email, ideally already attaching some further information (TB version, error console log).