How to use socks with Firefox
Firefox is the most powerfull and extendable browser in the world. So how to use socks with Firefox 3, is it hard to configure and use? No, it easy.
Run Firefox
Go to Edit → Preferences

Click Advanced and choose Network, then Settings

Choose Manual proxy configuration and place IP address to SOCKS Host, port to Port
After that choose SOCKS v4 for 4 version of socks or SOCKS v5 for 5 version of socks

Click OK
Enjoy using socks in Firefox
For comfortable socks switching and configuring you can use FoxyProxy extension for Firefox