I vaguely remember having that on a new user account that I created for a demo.

I think what I did back then was to go to about config and set browser.tabs.drawInTitlebar to 0

Not just window position. Also size, which screen, which virtual desktop, which activity, state (maximized, etc)

Strange video description. Contains advertisement for Proton Drive which does not even have a Linux client or any form of KDE integration