Ambi

Assuming that the firewall is on the inside of the Internet facing router and running OSPF to the router, then it does not matter much. It will deliver Internet bound traffic to the Internet facing router which will make the load balancing decisions.

As Jan suggests the common solution for doing Internet with 2 ISPs is to run BGP. There is a possibility of doing it with 2 default routes and also a possibility of doing Policy Based Routing so that most traffic went on one link and some identified traffic goes on the other link.

The previous suggestions are primarily oriented to outbound traffic. There is also a question of inbound traffic. How to do that is dependent on several aspects that we do not know - most especially what address space do you have, is it given to you by your ISP or is it provider independent addresses? Also dependent on whether you NAT your outbound traffic and on whether your Internet traffic is all things that you initiate from inside your network or do people in the Internet initiate traffic to you.

Doing Internet with a single ISP is relatively simple. Introducing a second ISP does introduce complications.

HTH

Rick