Sometimes the fact, that Mozilla apps are not integrated into OS X well enough is a good thing: I am using SSH tunnel to my work corporate proxy server rather than buggy and slow Cisco VPN. After I get tunnel up, I fire up Firefox as my browser and Postbox as my mail client - both use localhost:8888 as a SOCKS proxy. Works brilliantly, and in the meantime system is still connected to the internet directly, so I can see my home network without any issues and experience no delay related to sending data through the VPN.
By the way, did I ever say - Postbox works really really well, and can be compared only with Apple’s native client? Well, it doesn’t use OS X’s system-wide proxy - but, as I just said, it’s not always that bad!