Open ~/.config/kwinrc in your favorite txt editor (Kate) and look for BorderlessMaximizedWindows=true. Set this to false and logout and back in.
I feel like this should be exposed in the Plasma settings panel but it is not at this time.
It will fix most things but not Firefox. For that you’ll need to click the hamburger, More Tools, Customize Toolbar, and click to enable the Title Bar on the bottom left.