I just did an update to 3.27.6 on a trial site to again take a look at it before purchasing, and when viewing the site in FireFox Developer Edition, the site throws up the 'invalid license' popup.
It only happens when the dev toolbar is active and the window is set to responsive view - and - when the layout style is set for 'Sidebar'. If I exit responsive mode and refresh, I see no popup. If I stay in responsive mode and choose 'Sidebar right' or 'Toolbar Float', etc., there's no problem with popups.
A screengrab while in responsive mode, and set to Sidebar: