Hi Korwin, the extension can be set to automatically load the green "Keep Open" page in a new window, or you can click its toolbar button to open that page. There you will find the buttons to activate protection in that window (the buttons are in the Keep Open page, not on the tab context menu -- see https://addons.cdn.mozilla.net/user-media/previews/full/220/220130.png).

As far as I can tell, the extension is working in Firefox 93.0b7 (Developer Edition). If you are not even seeing the toolbar button, make sure the extension has permission to run in private windows (https://support.mozilla.org/kb/extensions-private-browsing).