Those who are still did not make switch from KDE3 to KDE 4 here is a small note on how to set /change default internet browser to firefox.

KMenu -> Control Center -> KDE Components -> Component Chooser -> Web Browser

Now tick the radio button "in the following browser" and enter the full path to firefox executable which can be obtained by running 'which firefox' from the command line. For example /usr/bin/firefox

Now you are all set and no restart is required.