Awesome and should be standard Thunderbird. As others note it doesn't work on a unified inbox, but it does fine on individual inboxes and that's good enough for me. I have a mailbox set aside for WiFi connection requests on our LAN and so it tends to fill up and I was finding a regular action was to mark all read using a right click, then Mark Folder Read. This saves me a click and navigate every time I do this, and is much appreciated.
I'm using it on 52.4 without any issues.
This review is for a previous version of the add-on (0.7.1).