Your netscape-comm-4.51-3.i386.rpm file requires libc.so.5 and libm.so.5 libraries. But Netscape 4.51 is glibc based and does not require libc5. I uninstalled libc5 , and Netscape works fine without them.
doh! It's from the libDynFonts.so, which will be removed in the next netscape release (4.51-5, or somesuch...)