Firefox moves window to the current desktop when opening links

Since one of the updates from around the holidays Firefox has started to move the whole window to the current desktop (KDE Plasma / kwin5) when I click on a link (e.g. in Emacs) instead of just opening a new tab where I put it. How do I get the old behaviour back? Achim. -- +<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+ Wavetables for the Terratec KOMPLEXER: http://Synth.Stromeko.net/Downloads.html#KomplexerWaves