I should have explained it better, I was asking about associating actions to the "compact" titlebar (or what's called). I guess I need to wait for a proper GTK3 "header bar", if it's planned at all...
Anyway thanks.
Edit: in Chrome/Chromium works as I expected, I think I should file a bug then.
I believe this isn’t enabled for KDE Plasma.
If you like a separate title bar like the old version of Firefox, then use the hamburger menu, choose “customise” and check the box for the title bar.