Don't forget that AV programs don't just help protect against viruses,
but also against trojans, and there's a lot more of those being
written for Firefox these days ...

Whereas there's no guarantee that you'll be 100% protected if you've
got even an up to date anti-virus program, it's still always better to
have one than not!

I understand that Linux machines are now becoming the Holy Grail for
hackers who can claim lots of status for succeeding in hacking one, so
go for the av, go for the firewall, go for the anti-spyware, go for as
much security as you can get, make sure it's set up properly, and kept
fully up to date. Only connect to the net when you need to, rather
than leaving it connected 24/7, and keep monitoring your logfiles and
such, just to make sure you're still safe.

Will you be 100% protected? No. Nothing is ever 100% guaranteed. If
somebody wants to get you enough, they will. The only way to keep your
computer 100% safe from internet-based hackers is not to ever connect
it up to the net.

One thing that puzzles me, however, is how these 6 other pc's you have
are now trash. You should be able to wipe, reformat & rebuild them (if
necessary, changing the network card(s), so that your MAC addresses
are different ...