Did you use YaST to install Firefox? If you did it should be in the program menu. Now if you right click it there you will see “Add to favorites” option. If you did not use YaST, my suggestion will be use it and things will be much more simple and safer.
To add to the panel you can just drag and drop from program menu.
Yes, you can install KDE using YaST. Make sure that KDE repositories are added and install KDE base system. I will suggest to install KDE 3.5 for now. Once you are conformable with it, then you can try 4.0 or 4.x.
Though I use bot KDE and gnome, I feel more comfortable using KDE in openSUSE. Buy its a personal choice. Let us know how things are going.