I really like the vertical tabs. For years I've struggled with trying to determine which tab goes with which page of a site as the width of individual tabs grows small. Since I am using a screen that is wider than it is tall, this seems to work as a ...
I would also love to see this support come to Thunderbird. Between email, IRC, RSS, and hopefully ActivityPub, Thunderbird could be my go-to for communication and subscription protocols and I won't have to have a web app open in Firefox.
