I followed the how-to on http://sixrevisions.com/web-development/how-to-debug-php-using-firefox-with-firephp/ but ran into the following problems.
I had problems with this modem (spontaneously delivered by Ziggo Netherlands) because the domain name resolving did not work properly.
It did not matter if I used Ziggo’s own domain name servers or the Google domain name servers.
The problem was persistent for about 70% of the time.
After calling with the help desk someone switched off the router capabilities of the modem which resolved my problem as I use my own router for my network anyway.
Now I have my WAN IP address directly on my router.