Rated 4 out of 5 stars

It functions with "New Mail" and "Forward" perfectly, but not with "Reply", which is much more important, i think. I mostly mention someone, when I reply to an E-Mail. That should be fixed.

I debugged it a bit and there are two problems:
- You need to click into the editor area to make it work. Same is true for "New Mail" and "Forward" by the way. If you navigate via keyboard, the extension will not receive key strokes.
- Even if you did click into the editor area, the popup will be rendered at a wrong position (mostly likely).

The second problem is easy to fix, but the first seems to me like a bug in Thunderbird. The event listeners simply to not fire until you clicked into the editor area.