I have two computers - CompA and CompB. I currently have an ssh tunnel set up from A to B using a command like: "ssh -D 8080 -f -C -q -N user@myserver.com"

Then on CompA, I open firefox and change it to use a socks proxy at 127.0.0.1. This works just fine - when I visit sites like www.whatismyipaddress.com, it shows CompB's ip address.

Here's my problem: CompB has a bunch of external IP addresses. I'd like to be able to change which IP address it uses when I connect to the proxy. This way, I can use any of CompB's IP addresses when browsing the web from CompA. How is this possible?

I hope that makes sense...

well the source address will just be assigned as the interface used according to the routing table on compB

You would need to change the routing path used from the remote computer to reach the site you are connecting from. As acid_kewpie said, the IP of your webtraffic is decided by the routing table on the remote end.