If on Linux/OSX, SSH to some machine with the -D option, which makes a Dynamic SOCKS5 proxy.
$ ssh -D 1080 some.machine.com
Either way, set Firefox to use your tunnel:
First, go to options | Advanced | Network tab | Connection settings:
Then in the proxy dialog, select to "Manual" and blank out everything but "SOCKS Host", which is set to "127.0.0.1:1080":
You should be able to surf through the tunnel now. Check http://whatismyipaddress.com/ and verify that the IP shown is the one from the machine you connected to via SSH.
If you tend to do this a lot, consider installing the SwitchProxy extension for Firefox, which will let you change proxies in two clicks: