Review for Get All Mail Button for TB78++ by Steve Roberts
Rated 5 out of 5 stars
Does exactly what you would expect, why Thunderbird does not have this basic option is a mystery.
Also if like me you don't like the green arrows, right click and 'manage' then you have several colour/ grey / blank options... looks much better!
To create your own collections, you must have a Mozilla Add-ons account.