check if it is listed in the start menu items. It should be under internet. If it isn't, run settings->menu updating tool & see if KDE's wizardry can find it. If that works, the easiest way to create a desktop icon is to drag'n'drop copy the menu itm to the desktop.

If menu update can't create the icon, you will have to create it yourself. First, find the executeable file, it should be in /usr/bin or possibly /usr/sbin. Then right click on the desktop, 'create new->file->link to application' and set up a link to the firefox executeable. Rename it 'Firefox', find a suitable icon & your done. You can change the icon &/or any other properties of the link later by rt click on it & select properties.