ssh -D port host
ssh -D 3125 siona.nibble.bz
Then configure Firefox (Edit - Preferences - Connection Settings) with a SOCKS proxy of localhost, port 3125 like this:
The best test is to go to What Is My IP? and refresh the page with the proxy disabled/enabled and verify the IP address changes.
Now for shits and giggles, you use -f and -N with SSH to background the ssh process (the -f) without running any remote command (the -N) like this:
ssh -fN -D 3125 siona.nibble.bz
This will leave your SOCKS proxy in the background so you can close your terminal and still surf through the proxy.
Hooray for bypassing crappy firewalls and HTTP proxies!