redundant backbone connections

I have two internet backbone connections, each connected to it's own firewall (Nokia IP330s). Only 1 connection is needed at a time (however, using both is fine if it's easy to configure). How would I go about configuring a switch or something to provide this redundancy? I have some extra switches to spare (cisco 2950s). I tried looking into trunking, and am not sure how to configure it. I also looked into spanning tree, and I know it prevents loops, but I'm not sure if it works for this.
Someone please help, it's urgent.

Thanks in advance.