Gustavo, all Netscape releases from 6.0 onward are "Mozilla-derived." However, you really want to run Mozilla itself. To begin with, as Don points out, Netscape 6.1 is based on an earlier and more troublesome version of Mozilla. (Unlike Netscape, Mozilla releases a new version every 6 weeks or so.) Also, Netscape adds a lot of cruft on top of Mozilla. Now, if you happen to like that particular cruft, it's fine in principle to use Netscape. But if you think about the fact that Netscape 6.1 is new cruft on top of a not-quite-1.0-release Mozilla, you have to start worrying about stability.
