Firefox was starting really slow, taking almost 2 minutes before the startup screen was disabled. As suggested by Redhat bug 1290894 disabling the network connection will Firefox start at normal speed again. How-ever this is rather useless :-).
Running wireshark and strace revealed an interesting observation. Firefox (like many other programs) is first trying to resolve the machine hostname. Since this was not properly configured on my machine, it was not able to find the right answer.
Fixing it by adding the entry file:/etc/hosts
echo "127.0.0.1 $(hostname)" >> /etc/hosts