Rated 5 out of 5 stars

Does exactly what it says. Would be nice if it scaled text without needing to reopen the window (this is what would lead new users to think it doesn't work), but it only needs to be done once, so - excellent. Thank you very much.

Rated 2 out of 5 stars

Great idea but sadly doesn't work in v128 (Betterbird fork)
It would be good to have this for reading messages too, if it worked, since I can use keystrokes to zoom in the message editor, not when reading messages.

This review is for a previous version of the add-on (1.1). 

Why do you rate so badly if you don't use the add-on as intended? It is an AddOn for Thunderbird, not for Betterbird. And it is clearly described that it is intended for the Composer. The read view for emails can be controlled in the default settings.

Tested meanwhile in Betterbird. AddOn works perfectly.