Ok. I had been told it could be some issue in Kwin not really terminating firefox-bin process when clicking on the close button, that's why I thought it was a window manager issue, but there ya'll are, guys, always a source of knowledge, ;) . Purged gtk-qt-engine and the problem gone. Now IW looks ugly like a tooth-ache, but I'll try Valerio's recommendation. Thanks a lot Gombang, Modestas, Vale, :)