Opera Playground

Thunderbird is the first client I've seen with a feature for moving in the history of messages that you've viewed.

The first client?
We've used such a function for a long time, with Opera Mail. We can use it even with Mouse Gestures! The author doesn't know it, it seems.

[Edit at Sunday Morning, 28th of Jan.] As someone pointed out at the comments there, Opera just moves from one view to another view, it doesn't remember the exact position of what you opened in the view. That was my misunderstanding, sorry for that.
I put left fingers on Z and X kyes, right fingers on up and down arrow keys, and also use spacebar, when I read mails BTW.