Youll find most anti-virus programs on linux arent really anti-virus programs like you are used to in MS. I have never run one because I dont run a mail server, and that is what a lot of them are, they scan mail going through a server for viruses.
So few viruses are written for Linux for two reasons mainly(Well ok three depending on how you count it). One being it has such a small user bae compared to Windows, and that user base is typically the more technically savy people that it doesnt do much good to write one. Two is because if you have used linux for any length of time, you KNOW not to run anything as root unless you have to, therefore most viruses could at most on a well setup system only affect one user, and not really do much permanant harm.
Typically Firewall programs in linux are easy to come by, stable, and good, although maybe a bit difficult to figure out at first. And oh yea they are free too which doesnt hurt
Most of the time most Distros will have at least one program with em, sometimes several different firewalls. Gotta Love Linux for networking.
Thunderbird and Firefox should work fine on linux, though if you are looking for for the Outlook type functionality you might try Evolution(Ximian product). It is comparable if you need the added things such as a calendar, if you are only looking for email thunderbird is not a bad choice, though you may want to wait untill it is at least in its preview release stages(Last I think it is still in Beta 0.8 stage I believe)
I actually use mozilla right now, but I am in the process of learning to build my own Distro so I stop spending money on Suse and when I do I will be centering the desktop around E17(If it ever comes out) and firefox(With its calendar extension installed as long as it does what I need) and thunderbird. Between those and openoffice I have pretty much all the office programs I need and pretty decent ones at that. Add on to that Samba to allow networking with linux, and the web scripts to browse samba shares that exist and most of my networking will be done through Firefox, and file management through E. Dang I love how flexible that program is.
If you want basic Multimedia functionality, I would suggest XMMS for audio, and Xine for Video(Though truthfully you MIGHT be able to get away with just Xine)
I dont have any experience with Knoppix, I have tried Distros from Mandrake, Red Hat, and Suse myself. Out of those I recommend Suse for people just starting out with a decent computer system, red hat for those with a less than decent computer system, and at the worst Mandrake. I have read posts from others that had problems with Suse where they didnt with mandrake, however my opinion has been the opposite, I have never gotten a Mandrake install to work correctly(Not mounting HDs or Removeable Media is a SLIGHT problem
so I think it may just depend on your luck with it. If you are just starting in linux get the Off the shelf Distro with the Tech Support and be ready to use it, however if you know your way around it you might be better off just downloading it.