Rated 2 out of 5 stars

Great idea, but can't get it to work. After installing on tb 78.6.1, I see "++" in middle pane, but can't click on it (prob due to "message header toolbar customize" add on I have. Why can't it be put in the Mail Toolbar? Or right click context menu? (this on fedora 28).

This review is for a previous version of the add-on (2.0). 

yes, right click context menu is a good idea. I will add that and also keyboard activation.

If ++ doesn't work due to the other addon - why give 2 stars only to us, if it is them?
The button is inserted by the new TB addon system, not by us. I will send them a bug report.