diff --git a/libs/mozilla.build.sh b/libs/mozilla.build.sh
index d8ce24fce9..1f3c03bb71 100755
--- a/libs/mozilla.build.sh
+++ b/libs/mozilla.build.sh
@@ -1,3 +1,11 @@
-cd nsprpub && ./configure && make
+arch=`uname -m`
+
+opts=""
+if [ $arch = "x86_64" ] ; then
+opts="--enable-64bit"
+fi
+
+cd nsprpub && ./configure $opts && make
+
 cd ../js/src && JS_THREADSAFE=1 OTHER_LIBS="-L../../../mozilla/nsprpub/dist/lib" INCLUDES="-I../../../mozilla/nsprpub/dist/include/nspr"  make -f Makefile.ref `find . -name libjs.a`