Just Worked Rated 5 out of 5 stars
Just did what it said: As I expand the width of the Thunderbird window, it automatically switches from "wide" view (with the message pane below the inbox) to "vertical" view (with the message pane to the right of the inbox, occupying the full height), and vice-versa.
Only one configuration parameter: "Threshold", being the width (in pixels) at which the layout will switch to vertical mode. The default value of 1300 seems right to me.
My ideal plug-in: A single purpose, minimal configuration, does what it says. Please don't add anything else!