General Questions

I can see in the Status > Interfaces menu, on the web gui, that I've two Interfaces.

The Wan is

Media 10baseT/UTP <half-duplex>  

The LAN is

Media 100baseTX <full-duplex>  

I suppose that monowall has an autodetection of the ethernet speed.

But how can I do to set a static speed?

My ISP requires me to set a static speed, and not an Autonegotiation.

How can I do it with monowall?

from http://doc.m0n0.ch/handbook/faq-hiddenopts.html

interfaces/(if)/media and interfaces/(if)/mediaopt
If you need to force your NIC to a specific media type (e.g. 10Base-T half duplex), you can use these two options. Refer to the appropriate FreeBSD manpage for the driver you're using to see which options are available (or run ifconfig -m).