Hi Paolo,
You have to edit the /etc/sysconfig/SuSEfirewall2 file to open ports permanently.
Assuming aMule runs on the firewall host, you have no internal network and use the default ports (4662, 4672), the relevant settings are:
# 1.) Should the Firewall run in quickmode?
FW_QUICKMODE="no"
# 2.) Which is the interface that points to the internet/untrusted networks?
FW_DEV_EXT="...whatever your internet interface is..."
# 9.) Which services ON THE FIREWALL should be accessible from .. the internet ...?
FW_SERVICES_EXT_TCP="4662"
FW_SERVICES_EXT_UDP="4665 4672"
If you use different ports, you surely will be able to figure it out yourself

It's also possible to use FW_QUICKMODE="yes", in this case set the FW_SERVICES_QUICK_TCP and FW_SERVICES_QUICK_UDP variables (but I haven't tried that variant).
Ciao!