I guess I should have closed out the issue :).  As it works out, I updated my system
from a snapshot to 
NetBSD-2.0 via /usr/src and anoncvs ... it seems that the upgrade did not delete
<sys/statvfs.h> (apparently 
now obsolete), and issues with configure which seemed to allow both statvfs.h _and_
statfs.h at the same
time .... is why firefox failed to build.   Removing the "offending" <sys/statvfs.h>
file fixed it for me :).

  Many thanks for all the help,
  - Julius.