To create a shortcut on the desktop in KDE, right click in some open space, then: Create New > File > Link to Application. Two tabs are important, General, and Application. In the General tab, you will need to fill in the name of the shortcut (eg, Firefox) plus you can choose the icon by clicking on the blue gear on the left hand side (for Firefox, choose Other Icons, then Browse to locate the icon you want to use). In the Application, click Browse to locate the Firefox executable. Save your changes and you're done.
FWIW when I was learning my way around KDE, this question drove me nuts for a couple of weeks. -- J.W.