1) I don't have to upgrade every six months due to the release of a security upgrade that should have been out five months ago.

2) I don't have to worry about the latest email virus because Netscape can be cleanly set up to not auto-open attachments- and to not execute Javascript in email. (Why support for Javascript was included in the email client in the first place is beyond me; but at least I control it!) The same goes for VBScript viruses.

3) I'm not vulnerable to ActiveX viruses, nor Code Red and most variants.

I still keep an antivirus scanner active and up-to-date on my system, because I know that there will someday be a virus that can take advantage of some known flaw in the software I'm using, and to scan downloaded executables.

There's also a very good reason I don't use Netscape 6.x and higher:
I consider its handling of local email folders to be badly BROKEN!

I tried installing 6.2 (IIRC) a while ago, and played with it, and found it usable. I then loaded up its email interface, and started moving things around, tried retrieving my IMAP mail, and so on. Everything *seemed* to work OK... but then I dropped back to 4.7 (which I still had installed) to see if the messages I had shuffled were visible in their new locations. Nope. (It also didn't have an option to use text-only buttons in the browser; the graphic-only ones are just funny-shaped squiggles- and still take up more space than the text!)

Netscape 6.2 refused to write new data to my local mail folders in their existing location. It would happily read them, and would apparently write data to them.... when in fact it was writing data to L:\winnt\profiles\kdeugau\Application Data\Mozilla\{some CLSID}\{etc...} Which I very explicitly did NOT want. Drive L: is my NT system drive. Personal data does NOT belong on the same drive as Windows.

(I've kept personal data and system files/programs separated onto different logical drives since DOS < 5.0; I WILL NOT use any software that refuses to put my data where I tell it to. No matter how good it might be otherwise.)

I haven't tried Mozilla, primarily because it's based on Netscape 6.x (or Netscape 6.x is based on it, same difference). And (except for in a few very rare cases) 4.7 is still working very well for me. I keep an old (3.0 or 4.0) IE around as a quick-and-dirty .JPG and .GIF viewer (and I'm thinking I should add .PNG to that list for 4.0) and as a "test" to make sure any website design I do looks at least halfway-reasonable with IE, but I don't visit outside websites with it.

I've tried Opera, but it was slow, occupied too much screen space with graphical buttons (I've got Netscape set to use text-only buttons), and generally didn't work well for me.