No product is free of security holes, Macs, Firefox or Linux.
However, some are more secure than others, and some are a LOT more secure than others.
Windows was built before the Internet existed.  PCs were designed to be standalone systems, the whole idea of user privledges is poorly implemented in XP meaning most people run as administrators so programs can do 
anything.  On the other hand, Mac OS X is based off of Unix which was designed with the internet in mind.  You run as a user in your own private world and can't touch the core of the operating system without a password prompt.  The same is true for Linux.  Also, OS X and Linux don't have tonnes of legacy code from the last decade to support every 10 year old program.  Legacy code by its very nature is full of security holes because there are tonnes of things that the developers could never have seen coming.
The other advantage firefox has over internet explorer is that it isn't integrated.  If IE is comprimised it can take down your whole system, its built into the kernel of windows xp, the very core of the operating system (you can't even uninstall it if you want to).  Firefox runs on its own, its not integrated with the operating system, and it is based on a much more simplistic architecture - meaning there are less chances for bugs.
The arguement that any operating system or program is entirely secure is BS.  However, there are no known viruses for OS X (there are a few proof of concept things, however all require a password prompt which should set off warning bells for any user and none have been distributed.  windows viruses can gain access to the root level of the OS) and far fewer exploits for firefox than for IE.  Security through obscurity is a nice arguement, but by now there should be at least one exploit that takes advantage of OS X because anybody who found it would be famous.  I am sure there are some exploits to be found in OS X, as in Linux...but I'm also sure there are a LOT fewer than there are in windows.
(i'm just rambling and typing what i think as i think it, so i hope i made my point but maybe i didn't....god i need sleep 

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Also, I think its a stretch to say its unfixable.  You'd be surprised what the open source community can do.  Besides, if MS can fix up xp and make vista secure than anything is possible!