I have a compuer with 2 nics that are connected respectively to two phisically separated network and there is no routing between the two networks. How can I set the internet traffic to go always on a nic and the intranet traffic to go always to the other one? Thanks a lot to whomever will answer me.

If the internet and LAN are separate subnets then it won't be an issue. Each NIC will have to be configured for it's own subnet. We would need to know what subnet's you're going to be using for the two separate networks in order answer more clearly.

Imagine that I have the network 10.10.10.0/24 (network A) and the network 192.168.0.0/24 (network B). Both networks can access to internet. Network A has low speed internet link and Network B has a high speed internet link. One nic is configured on network A and one nic is configured on the network B. I would like to use firefox to browse internet sites only through the network B (the faster one), and let all the other applications access the internet through the network A.

simple: don't provide a gateway entry for nic A

If I don't provide a gateway on nic b, how can I ensure that firefox will use the network b and not network a?

It will look for a gateway. If the only one available is for network b then that is what it will choose. Review my ammended answer above. You don't put the gateway entry on the network you don't want to use for internet.

Ok, really my network A is not a single subnet, but a lot of subnet routed from the gateway, if I remove the gateway, my intranet application won't work! Is there a "route command" that can identify the traffic not destinated to a particular domain? I would like to configure a static route that say: if the traffic go to *.mydomain.com pass through the network A, if not, pass through the network B.