Mozilla build updated my old gtk+ to "gtk+-1.2.10nb3".
  It took ages, but I assumed that this is what my latest
  pkgsrc tree required.

This is recent gtk 1.2.

  "gtk+-2.0.8.tar.gz"  another one!  I can live with building

This is gtk 2.0, which has different filenames; the two libraries
(with different APIs) coexist just fine.  Gnome2 stuff in general
needs gtk 2.0 and thus there is a transition period to glib2/gtk2 (and
ORBit2 and so on).

For example, one of my boxes has (with no ill effects):
gtk2+-2.0.6nb3      Gimp toolkit. Libraries for building X11 user interfaces
gtk+-1.2.10         Gimp toolkit. Libraries for building X11 user interfaces

        Greg Troxel <gdt@ir.bbn.com>