Selecting WiFi as "Default" Connection to use?

I’m connected to the internet via a wireless connection, but I need to hook up my machine to a physical LAN as well.

This physical LAN does not have internet access, and so I want it to use the wireless connection to access the interwebs instead. However, the browsers (namely Firefox) seem to default to going through the LAN connection.

any idea how I can set my browser to use the wireless instead?