Some questions about Mint

I'm largely new to Linux and have installed Mint 14.1 Cinnamon, so far I'm liking it a lot. I have a few questions though:

Why is the firewall disabled by default? Even if you enable, by default it just blocks inbound and allows outbound like win firewall, so I don't think it would cause issues.

I'm connected to an external monitor via VGA which is detected once connected and by default it is used as desktop extension. So far the only way to get the desktop on my external monitor and have a higher resolution than my notebook screen is to turn off the notebook display, but when I disconnect the VGA cable later, it doesn't turn it back on automatically, so I have to connect it again to turn it on manually. I have already enabled the latest Nvidia driver in Software Sources, it does fix the screen being off during boot-up but doesn't fix my issue. Any ideas?

My notebook has optical S/PDIF through the headphone output, but it isn't detected by default and Software Sources only shows drivers for my Nvidia GPU, not for any other device. It's from Realtek, are there drivers available to enable the digital output?

Of course I have also been reading a few articles on Linux security and wanted to try out Eset's AV, but the installer is a .linux file, when opening it says there is no application installed for executable files and asks me if I want to search for an application to open the file. How can I open it? UPDATE: Apparently I had to edit the file properties and allow executing file as program.

Is there any easy guide for AppArmor?