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By Security through Obscurity they mean that because Linux has such a low market share compared to Windows
You must be psychic. How do you know what EightPence means?
Security through obscurity means relying on non-standard programming practices and secrecy in a mistaken belief that attackers will be unable to figure out how to bypass security measures. It has nothing to do with market share, or how often an OS is attacked. Linux employs security by design, which is exactly the opposite. Since Linux and software that run on it are largely open source, potential security flaws can be easily spotted and corrected. There is no secrecy involved.
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I don't imagine a firewall is greatly important....
Really? Do you leave all the doors and windows on your house unlocked when you go away too? The more potential routes you have open into your system, the greater the chance that an attacker can find a way onto your system. A firewall is to keep out unwanted traffic.
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i don't use one
Unless you are some sort of a security professional that can share with the rest of us a solid argument for not using a firewall, then the fact that you choose to not should not be taken by any one to mean that it is ok to go without one.