java/jdk13 will fail to build if WITHOUT_PLUGIN is not specified and you have
x11-toolkits/qt30 installed.
qt30 installs the following useless header files:

  /usr/X11R6/include/npapi.h
  /usr/X11R6/include/npupp.h
  /usr/X11R6/include/jri.h
  /usr/X11R6/include/jritypes.h
  /usr/X11R6/include/jri_md.h

symlinking these to their counterparts in the mozilla subdirectory helps, but
java/jdk13 then fails in:

gmake[1]: Entering directory `/common/ports/java/jdk13/work/j2sdk1.3.1/ext/plugin/build/solaris'
gcc -Wall  -c -I/usr/local/include -O -D_GNU_SOURCE -DXP_UNIX  -DVERSION="\"1.3.1-p6-root-020509-18:39\"" -DDATE="\"May-09-2002\"" -DARCH="\"i386\"" -DRAPTOR_API -DNEW_STREAMING_API -I../../oji-plugin/include/solaris/navig4/nspr -I../../oji-plugin/include/solaris/navig4/xpcom -I../../oji-plugin/include/solaris/navig4/caps -I../../oji-plugin/include/solaris/navig4/js -I../../oji-plugin/include/solaris/navig4/raptor -I../../tmp/bsd/i386/CClassHeaders -I../../oji-plugin/include/solaris/navig4/oji -I../../oji-plugin/include/solaris/navig4/plugin -I../../oji-plugin/src/motif/common -I../../oji-plugin/src/motif/navig4 -I/usr/X11R6/include -I../../oji-plugin/src/motif/badapter -I../../../../src/share/javavm/export -I../../../../src/solaris/javavm/export -I/usr/X11R6/include  -I../../oji-plugin/src/share -o ../../tmp/bsd/i386/stubs.o ../../oji-plugin/src/motif/common/stubs.c
In file included from ../../../../src/share/javavm/export/jni.h:30,
                 from /usr/X11R6/include/jritypes.h:46,
                 from /usr/X11R6/include/jri.h:45,
                 from /usr/X11R6/include/npapi.h:79,
                 from ../../oji-plugin/src/motif/common/stubs.c:29:
../../../../src/solaris/javavm/export/jni_md.h:21: redefinition of `jint'
/usr/X11R6/include/jri_md.h:197: `jint' previously declared here
../../../../src/solaris/javavm/export/jni_md.h:23: redefinition of `jlong'
/usr/X11R6/include/jri_md.h:231: `jlong' previously declared here
../../../../src/solaris/javavm/export/jni_md.h:24: conflicting types for `jbyte'
/usr/X11R6/include/jri_md.h:191: previous declaration of `jbyte'
gmake[1]: *** [../../tmp/bsd/i386/stubs.o] Error 1
gmake[1]: Leaving directory `/common/ports/java/jdk13/work/j2sdk1.3.1/ext/plugin/build/solaris'
gmake: *** [plugin-all] Error 1
*** Error code 2

At this point, I realised trying to take on the source trees for Mozilla, QT3
and JDK13 would be a mostly futile effort. Hopefully, those more acquainted
with these sources might be able to determine cause and a resolution.
I'd like to fix this, but I need some guidance as to the correct course of
action.

Fix: 

Specify WITHOUT_PLUGIN=yes when building java/jdk13.
Get QT3 to DTRT with respect to the installation of header files, maybe?
Again, I'm happy to help fix this, but I need some guidance.
How-To-Repeat: 
cd /usr/ports/www/mozilla ; make install
cd /usr/ports/x11-toolkits/qt30 ; make install
cd /usr/ports/java/jdk13 ; make install