Auto Toggle Vertical Tab от Reguna

Check the width of currently focused Firefox window every 0.1 second:

1. If width <= user defined threshold, enable vertical tab
2. If width > user defined threshold, disable vertical tab

The user defined threshold is 1080 (in pixels) by default.
You may also choose to reverse this behavior, so that vertical tab is enabled when the window width is ABOVE the threshold (and vice versa).
You can edit these settings in the extension menu.

This extension is open source (GPLv3) and does not collect any data nor send any data to any server.
https://github.com/regunakyle/firefox-auto-vertical-tab
(You can also see why I use interval instead of window.onresize event listener in the Github README)

The app icon (created by Smashicons) is downloaded from flaticon.com for free.