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.
Someone knows if it's customizable in the config menu, searching the obvious "titlebar" reports only two not useful entries?
Anyway thanks.
Edit: in Chrome/Chromium works as I expected, I think I should file a bug then.
Could be that Ubuntu were already doing this and it has now made it upstream, I suppose.
I haven't used gnome in quite a while, so I'm not totally certain of what was before this change, but I wouldn't consider it problematic. I find that saving the extra 20 pixels at the top is good for the widescreen aspect ratio of most monitors.