I don't think you can do this within Firefox. If you do a 'firefox --help' on the command line there is no switch to specify a specific network interface.
You could work around that by creating a local proxy for one of the interfaces and use a custom profile for that one instance of Firefox and point it at the your new local proxy inside the profile.
Personally, i have achieved something like that. I used my laptop to pick up the wifi. Then, used the ethernet on the laptop to run to my xp desktop. the trick to it is one of the wifi connections has to be shared if i can remember right its the ethernet port. This can all be done by
-clicking the wifi symbol
-then at the bottom there's a ...