I have a wired ethernet connection and a tethered phone connection available.
Can I force Firefox to use only the ethernet connection and force Safari to use the tethered connection?
I'm running OS X 10.6 and don't mind mucking around.
Unless one of the interfaces is just for a specific LAN, you'll probably need to set up a proxy server like squid and configure one of the browsers to point to it. If you want to also split your DNS requests, you'll probably also need a local caching DNS server.
The problem is the default route on your machine is going ensure that all traffic outside the local network is going to be via that default route.
However, if there is a proxy server that is on the same network range of the tethered connection it might just be a matter of adding the proxy settings into safari to point to that.
Assuming the default route is ALWAYS via the wired ethernet, Firefox will route requests out of the LAN.