The following setting doesn’t appear to apply to Firefox or Librewolf for some reason:
Window Management → Window Behavior → Titlebar Actions → Double-click
I prefer double clicking the titlebar to minimize apps and I have to use workarounds to minimize them as I normally hide the minimize button. In Firefox I can force a properly working KDE titlebar to be shown at least, but then I basically have 2 titlebars for no good reason.
I also noticed that Lutris has the same problem. It seems related to GTK apps I guess? Is this a GTK window decoration problem in KDE?