How to close an ssh tunnel

I think you are mixing up two commands. Try to set Firefox to Socks5 at port 12345 and run:

ssh user@host -D 12345

From the SSH manual, 'man ssh':

-D [bind_address:]port
Specifies a local “dynamic” application-level port forwarding. ... Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh will act as a SOCKS server.

Your command user@host -L 12345:localhost:8080 -N forwards your local port 12345 to the remote host's port 8080. This would only work if there would actually be a proxy running on port 8080 on your remote host.

Most OS X users don't use "ssh GUIs" because OS X ships with a very good Terminal application. This is unlike Windows where the cmd.exe or even a PowerShell are not friendly command line interfaces to remote servers.

In OS X, you can just fire up Terminal.app and ssh from the command line with:

ssh <myhost>

You can use a more advanced terminal application like iTerm2 and it will allow you to save "profiles" that, when called up, automatically run a set of commands in the terminal for you. You can use these to automatically start ssh connections (which can be interactive terminal sessions or stunnels):

You can start and stop stunnels from the command line as well -- after all, these are just ssh connections that open a local port and forward requests to a remote server. For example:

ssh -f user@personal-server.com -L 2000:personal-server.com:25 -N

And you're done.

Private key authentication schemes are supported by the ssh command line that ships, default, with any OS X version.