[GREENFOOT-63] Pause-button doesn't work in applets running in firefox

After starting a greenfoot applet in firefox the pause-button can't be clicked with the mouse. Moving a greenfoot object in the world with the mouse, turns the pause button into the run button.

Using the keyboard (tab+enter) can be used to activate the pause button.

It works in Safari and the appletviewer. Works fine on linux too.