Comment # 6 on bug 1091592 from Mircea Kitsune

(In reply to Wolfgang Rosenauer from comment #5)

Thanks for clarifying. I thought this was a Firefox 59 specific issue at first.

I just installed Firefox 59.99 / 60.0b16 from
http://download.opensuse.org/repositories/mozilla:/beta/openSUSE_Tumbleweed I
can confirm the issue goes away in this version and the functionality is
exactly as expected: By default KWin continues acting as the window manager,
and the new builtin decoration is only used if you go into about:config and set
browser.tabs.drawInTitlebar to true. As long as that option remains set to
false by default (especially for KDE users) there should be no more issues in
the future.