if you press the multimedia play button it starts/pauses e.g. amarok, but sometimes also youtube.
in former times it just paused amarok (when I was a little boy)

Is there a way to get this behaviour back? Or how is the focus of the button decided? (e.g. if I minimize youtube the button is still focused on youtube...)

I suggest using the Amarok controls for Amarok instead of the Media Player which will halt whatever is playing...

When you were a little boy I guess the Plasma Media Player did not exist...