In the live system from the Simply Linux 6.0.1RC2 DVD, pressing the "web browser" button on the panel doesn't start any browser the first time (right after booting). But after Firefox has already been run once, the button works.
I can't reproduce this. Can you run command exo-open --launch WebBrowser in terminal? It is exactly what that launcher does. May be you just need wait a while.
I can't reproduce this.