Out of curiosity I booted up the latest Manjaro KDE iso in Virtualbox. I was surprised to see that even under the Plasma session, I get the GTK titlebar by default. Although my preference may alternate, I might prefer this as it’s the only way to get tabs in the titlebar for space efficiency. Was wondering which setting controls it so I can choose whether to use the Qt or GTK titlebar.
This is an option in Firefox, but this is also the DEFAULT so why do you have a title bar I don’t know.
(//EDIT: maybe it is the menu bar that is hidden by default, not sure, whatever…)
Right click next to a tab and select Customize Toolbar, then un-tick Title Bar at bottom left.
No title bar, and no menu, you have the result in first image.
PS: Firefox being a GTK app, I don’t know what you’re talking about for the QT title bar.