1. Open 2 windows of the file manager, Dolphin.
2. In one browse to the Desktop folder. In the other browse to this folder: /usr/share/applications/
3. Copy the Firefox and Thunderbird launchers from the applications folder to the Desktop folder.
4. For Dolphin, there is a folder /usr/share/applications/kde4/ where you will find the KDE4 applications... Copy the Dolphin launcher from there.
I just tried it... you can simply drag and drop from the menu right into the folder view plasmoid (that box).