diff --git a/CLOBBER b/CLOBBER
index 052f8159410..091c0155fc6 100644
--- a/CLOBBER
+++ b/CLOBBER
@@ -22,4 +22,4 @@
 # changes to stick? As of bug 928195, this shouldn't be necessary! Please
 # don't change CLOBBER for WebIDL changes any more.
 
-Clobber for bug 1022262.
+Clobber to make sure any residual damage from bug 975011 is gone.
diff --git a/aclocal.m4 b/aclocal.m4
index e5fdb4a0aa8..1c7296eea46 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -11,7 +11,6 @@ builtin(include, build/autoconf/toolchain.m4)dnl
 builtin(include, build/autoconf/ccache.m4)dnl
 builtin(include, build/autoconf/wrapper.m4)dnl
 builtin(include, build/autoconf/nspr.m4)dnl
-builtin(include, build/autoconf/nspr-build.m4)dnl
 builtin(include, build/autoconf/nss.m4)dnl
 builtin(include, build/autoconf/pkg.m4)dnl
 builtin(include, build/autoconf/codeset.m4)dnl
diff --git a/build/autoconf/nspr-build.m4 b/build/autoconf/nspr-build.m4
deleted file mode 100644
index 631f69e8f03..00000000000
--- a/build/autoconf/nspr-build.m4
+++ /dev/null
@@ -1,269 +0,0 @@
-dnl This Source Code Form is subject to the terms of the Mozilla Public
-dnl License, v. 2.0. If a copy of the MPL was not distributed with this
-dnl file, You can obtain one at http://mozilla.org/MPL/2.0/.
-
-AC_DEFUN([MOZ_CONFIG_NSPR], [
-
-ifelse([$1],,define(CONFIGURING_JS,yes))
-
-dnl Possible ways this can be called:
-dnl   from toplevel configure:
-dnl     JS_STANDALONE=  BUILDING_JS=
-dnl   from js/src/configure invoked by toplevel configure:
-dnl     JS_STANDALONE=  BUILDING_JS=1
-dnl   from standalone js/src/configure:
-dnl     JS_STANDALONE=1 BUILDING_JS=1
-
-dnl ========================================================
-dnl = Find the right NSPR to use.
-dnl ========================================================
-MOZ_ARG_WITH_STRING(nspr-cflags,
-[  --with-nspr-cflags=FLAGS
-                          Pass FLAGS to CC when building code that uses NSPR.
-                          Use this when there's no accurate nspr-config
-                          script available.  This is the case when building
-                          SpiderMonkey as part of the Mozilla tree: the
-                          top-level configure script computes NSPR flags
-                          that accomodate the quirks of that environment.],
-    NSPR_CFLAGS=$withval)
-MOZ_ARG_WITH_STRING(nspr-libs,
-[  --with-nspr-libs=LIBS   Pass LIBS to LD when linking code that uses NSPR.
-                          See --with-nspr-cflags for more details.],
-    NSPR_LIBS=$withval)
-
-ifdef([CONFIGURING_JS],[
-    MOZ_ARG_ENABLE_BOOL(nspr-build,
-[  --enable-nspr-build     Configure and build NSPR from source tree],
-        MOZ_BUILD_NSPR=1,
-        MOZ_BUILD_NSPR=)
-])
-
-if test -z "$BUILDING_JS" || test -n "$JS_STANDALONE"; then
-  _IS_OUTER_CONFIGURE=1
-fi
-
-MOZ_ARG_WITH_BOOL(system-nspr,
-[  --with-system-nspr      Use an NSPR that is already built and installed.
-                          Use the 'nspr-config' script in the current path,
-                          or look for the script in the directories given with
-                          --with-nspr-exec-prefix or --with-nspr-prefix.
-                          (Those flags are only checked if you specify
-                          --with-system-nspr.)],
-    _USE_SYSTEM_NSPR=1 )
-
-if test -z "$BUILDING_JS"; then
-    JS_THREADSAFE=1
-fi
-
-JS_POSIX_NSPR=unset
-ifdef([CONFIGURING_JS],[
-    if test -n "$JS_STANDALONE"; then
-      case "$target" in
-        *linux*|*darwin*|*dragonfly*|*freebsd*|*netbsd*|*openbsd*)
-          if test -z "$_HAS_NSPR" && test "$JS_THREADSAFE"; then
-            JS_POSIX_NSPR_DEFAULT=1
-          fi
-          ;;
-      esac
-    fi
-
-    MOZ_ARG_ENABLE_BOOL(posix-nspr-emulation,
-[  --enable-posix-nspr-emulation
-                          Enable emulation of NSPR for POSIX systems],
-    JS_POSIX_NSPR=1,
-    JS_POSIX_NSPR=)
-])
-
-dnl Pass at most one of
-dnl   --with-system-nspr
-dnl   --with-nspr-cflags/libs
-dnl   --enable-nspr-build
-dnl   --enable-posix-nspr-emulation
-
-AC_MSG_CHECKING([NSPR selection])
-nspr_opts=
-which_nspr=default
-if test -n "$_USE_SYSTEM_NSPR"; then
-    nspr_opts="x$nspr_opts"
-    which_nspr="system"
-fi
-if test -n "$NSPR_CFLAGS" -o -n "$NSPR_LIBS"; then
-    nspr_opts="x$nspr_opts"
-    which_nspr="command-line"
-fi
-if test -n "$MOZ_BUILD_NSPR"; then
-    nspr_opts="x$nspr_opts"
-    which_nspr="source-tree"
-fi
-if test "$JS_POSIX_NSPR" = unset; then
-    JS_POSIX_NSPR=
-else
-    nspr_opts="x$nspr_opts"
-    which_nspr="posix-wrapper"
-fi
-
-if test -z "$nspr_opts"; then
-    if test -z "$BUILDING_JS"; then
-      dnl Toplevel configure defaults to using nsprpub from the source tree
-      MOZ_BUILD_NSPR=1
-      which_nspr="source-tree"
-    else
-      dnl JS configure defaults to emulated NSPR if available, falling back
-      dnl to nsprpub.
-      if test -n "$JS_THREADSAFE"; then
-          JS_POSIX_NSPR="$JS_POSIX_NSPR_DEFAULT"
-          if test -z "$JS_POSIX_NSPR"; then
-             MOZ_BUILD_NSPR=1
-             which_nspr="source-tree"
-          else
-             which_nspr="posix-wrapper"
-          fi
-      else
-          which_nspr="none"
-      fi
-   fi
-fi
-
-if test -z "$nspr_opts" || test "$nspr_opts" = x; then
-    AC_MSG_RESULT($which_nspr)
-else
-    AC_MSG_ERROR([only one way of using NSPR may be selected. See 'configure --help'.])
-fi
-
-AC_SUBST(MOZ_BUILD_NSPR)
-
-if test -n "$BUILDING_JS"; then
-  if test "$JS_POSIX_NSPR" = 1; then
-    AC_DEFINE(JS_POSIX_NSPR)
-  fi
-  AC_SUBST(JS_POSIX_NSPR)
-fi
-
-# A (sub)configure invoked by the toplevel configure will always receive
-# --with-nspr-libs on the command line. It will never need to figure out
-# anything itself.
-if test -n "$_IS_OUTER_CONFIGURE"; then
-
-if test -n "$_USE_SYSTEM_NSPR"; then
-    AM_PATH_NSPR($NSPR_MINVER, [MOZ_NATIVE_NSPR=1], [AC_MSG_ERROR([you do not have NSPR installed or your version is older than $NSPR_MINVER.])])
-fi
-
-if test -n "$MOZ_NATIVE_NSPR" -o -n "$NSPR_CFLAGS" -o -n "$NSPR_LIBS"; then
-    _SAVE_CFLAGS=$CFLAGS
-    CFLAGS="$CFLAGS $NSPR_CFLAGS"
-    AC_TRY_COMPILE([#include "prtypes.h"],
-                [#ifndef PR_STATIC_ASSERT
-                 #error PR_STATIC_ASSERT not defined or requires including prtypes.h
-                 #endif],
-                ,
-                AC_MSG_ERROR([system NSPR does not support PR_STATIC_ASSERT or including prtypes.h does not provide it]))
-    AC_TRY_COMPILE([#include "prtypes.h"],
-                [#ifndef PR_UINT64
-                 #error PR_UINT64 not defined or requires including prtypes.h
-                 #endif],
-                ,
-                AC_MSG_ERROR([system NSPR does not support PR_UINT64 or including prtypes.h does not provide it]))
-    CFLAGS=$_SAVE_CFLAGS
-elif test -z "$JS_POSIX_NSPR" -a -n "$JS_THREADSAFE"; then
-    if test -z "$LIBXUL_SDK"; then
-        NSPR_CFLAGS="-I${LIBXUL_DIST}/include/nspr"
-        if test -n "$GNU_CC"; then
-            NSPR_LIBS="-L${LIBXUL_DIST}/lib -lnspr${NSPR_VERSION} -lplc${NSPR_VERSION} -lplds${NSPR_VERSION}"
-        else
-            NSPR_LIBS="${LIBXUL_DIST}/lib/nspr${NSPR_VERSION}.lib ${LIBXUL_DIST}/lib/plc${NSPR_VERSION}.lib ${LIBXUL_DIST}/lib/plds${NSPR_VERSION}.lib "
-        fi
-    else
-        NSPR_CFLAGS=`"${LIBXUL_DIST}"/sdk/bin/nspr-config --prefix="${LIBXUL_DIST}" --includedir="${LIBXUL_DIST}/include/nspr" --cflags`
-        NSPR_LIBS=`"${LIBXUL_DIST}"/sdk/bin/nspr-config --prefix="${LIBXUL_DIST}" --libdir="${LIBXUL_DIST}"/lib --libs`
-    fi
-fi
-
-AC_SUBST(NSPR_CFLAGS)
-AC_SUBST(NSPR_LIBS)
-
-NSPR_PKGCONF_CHECK="nspr"
-if test -n "$MOZ_NATIVE_NSPR"; then
-    # piggy back on $MOZ_NATIVE_NSPR to set a variable for the nspr check for js.pc
-    NSPR_PKGCONF_CHECK="nspr >= $NSPR_MINVER"
-
-    _SAVE_CFLAGS=$CFLAGS
-    CFLAGS="$CFLAGS $NSPR_CFLAGS"
-    AC_TRY_COMPILE([#include "prlog.h"],
-                [#ifndef PR_STATIC_ASSERT
-                 #error PR_STATIC_ASSERT not defined
-                 #endif],
-                ,
-                AC_MSG_ERROR([system NSPR does not support PR_STATIC_ASSERT]))
-    CFLAGS=$_SAVE_CFLAGS
-fi
-AC_SUBST(NSPR_PKGCONF_CHECK)
-
-fi # _IS_OUTER_CONFIGURE
-
-])
-
-AC_DEFUN([MOZ_SUBCONFIGURE_NSPR], [
-
-if test -z "$MOZ_NATIVE_NSPR"; then
-    ac_configure_args="$_SUBDIR_CONFIG_ARGS --with-dist-prefix=$MOZ_BUILD_ROOT/dist --with-mozilla"
-    if test -z "$MOZ_DEBUG"; then
-        ac_configure_args="$ac_configure_args --disable-debug"
-    else
-        ac_configure_args="$ac_configure_args --enable-debug"
-    fi
-    if test "$MOZ_OPTIMIZE" = "1"; then
-        ac_configure_args="$ac_configure_args --enable-optimize"
-    elif test -z "$MOZ_OPTIMIZE"; then
-        ac_configure_args="$ac_configure_args --disable-optimize"
-    fi
-    if test -n "$HAVE_64BIT_OS"; then
-        ac_configure_args="$ac_configure_args --enable-64bit"
-    fi
-    if test -n "$USE_ARM_KUSER"; then
-        ac_configure_args="$ac_configure_args --with-arm-kuser"
-    fi
-    # A configure script generated by autoconf 2.68 does not allow the cached
-    # values of "precious" variables such as CFLAGS and LDFLAGS to differ from
-    # the values passed to the configure script. Since we modify CFLAGS and
-    # LDFLAGS before passing them to NSPR's configure script, we cannot share
-    # config.cache with NSPR. As a result, we cannot pass AS, CC, CXX, etc. to
-    # NSPR via a shared config.cache file and must pass them to NSPR on the
-    # configure command line.
-    for var in AS CC CXX CPP LD AR RANLIB STRIP; do
-        ac_configure_args="$ac_configure_args $var='`eval echo \\${${var}}`'"
-    done
-    # A configure script generated by autoconf 2.68 warns if --host is
-    # specified but --build isn't. So we always pass --build to NSPR's
-    # configure script.
-    ac_configure_args="$ac_configure_args --build=$build"
-    ac_configure_args="$ac_configure_args $NSPR_CONFIGURE_ARGS"
-
-    # Save these, so we can mess with them for the subconfigure ..
-    _SAVE_CFLAGS="$CFLAGS"
-    _SAVE_CPPFLAGS="$CPPFLAGS"
-    _SAVE_LDFLAGS="$LDFLAGS"
-
-    if test -n "$MOZ_LINKER" -a "$ac_cv_func_dladdr" = no ; then
-      # dladdr is supported by the new linker, even when the system linker doesn't
-      # support it. Trick nspr into using dladdr when it's not supported.
-      export CPPFLAGS="-include $_topsrcdir/mozglue/linker/dladdr.h $CPPFLAGS"
-    fi
-    export LDFLAGS="$LDFLAGS $NSPR_LDFLAGS"
-    export CFLAGS="$CFLAGS $MOZ_FRAMEPTR_FLAGS"
-
-    # Use a separate cache file for NSPR since it uses autoconf 2.68.
-    _save_cache_file="$cache_file"
-    cache_file=$_objdir/nsprpub/config.cache
-
-    AC_OUTPUT_SUBDIRS(nsprpub)
-
-    # .. and restore them
-    cache_file="$_save_cache_file"
-    CFLAGS="$_SAVE_CFLAGS"
-    CPPFLAGS="$_SAVE_CPPFLAGS"
-    LDFLAGS="$_SAVE_LDFLAGS"
-
-    ac_configure_args="$_SUBDIR_CONFIG_ARGS"
-fi
-
-])
diff --git a/configure.in b/configure.in
index 423840ed753..262eadc2d60 100644
--- a/configure.in
+++ b/configure.in
@@ -53,7 +53,6 @@ dnl ========================================================
 MOZJPEG=62
 MOZPNG=10610
 NSPR_VERSION=4
-NSPR_MINVER=4.10.3
 NSS_VERSION=3
 
 dnl Set the minimum version of toolkit libs used by mozilla
@@ -132,7 +131,7 @@ EOF
   exit 1
   break
 fi
-MOZ_BUILD_ROOT=`pwd -W 2>/dev/null || pwd`
+MOZ_BUILD_ROOT=`pwd`
 
 MOZ_PYTHON
 
@@ -2201,6 +2200,7 @@ ia64*-hpux*)
 
     case "$host" in
     *-mingw*)
+        MOZ_BUILD_ROOT=`cd $MOZ_BUILD_ROOT && pwd -W`
         if test -n "$L10NBASEDIR"; then
             L10NBASEDIR=`cd $L10NBASEDIR && pwd -W`
         fi
@@ -3443,7 +3443,47 @@ MOZ_ARG_WITH_BOOL(system-libxul,
 [  --with-system-libxul    Use system installed libxul SDK],
     SYSTEM_LIBXUL=1)
 
-MOZ_CONFIG_NSPR()
+dnl ========================================================
+dnl = If NSPR was not detected in the system,
+dnl = use the one in the source tree (mozilla/nsprpub)
+dnl ========================================================
+MOZ_ARG_WITH_BOOL(system-nspr,
+[  --with-system-nspr      Use system installed NSPR],
+    _USE_SYSTEM_NSPR=1 )
+
+if test -n "$_USE_SYSTEM_NSPR"; then
+    AM_PATH_NSPR(4.10.6, [MOZ_NATIVE_NSPR=1], [AC_MSG_ERROR([your don't have NSPR installed or your version is too old])])
+fi
+
+if test -n "$MOZ_NATIVE_NSPR"; then
+    _SAVE_CFLAGS=$CFLAGS
+    CFLAGS="$CFLAGS $NSPR_CFLAGS"
+    AC_TRY_COMPILE([#include "prtypes.h"],
+                [#ifndef PR_STATIC_ASSERT
+                 #error PR_STATIC_ASSERT not defined or requires including prtypes.h
+                 #endif],
+                [MOZ_NATIVE_NSPR=1],
+                AC_MSG_ERROR([system NSPR does not support PR_STATIC_ASSERT or including prtypes.h does not provide it]))
+    AC_TRY_COMPILE([#include "prtypes.h"],
+                [#ifndef PR_UINT64
+                 #error PR_UINT64 not defined or requires including prtypes.h
+                 #endif],
+                [MOZ_NATIVE_NSPR=1],
+                AC_MSG_ERROR([system NSPR does not support PR_UINT64 or including prtypes.h does not provide it]))
+    CFLAGS=$_SAVE_CFLAGS
+else
+    if test -z "$LIBXUL_SDK"; then
+        NSPR_CFLAGS="-I${LIBXUL_DIST}/include/nspr"
+        if test -n "$GNU_CC"; then
+            NSPR_LIBS="-L${LIBXUL_DIST}/lib -lnspr${NSPR_VERSION} -lplc${NSPR_VERSION} -lplds${NSPR_VERSION}"
+        else
+            NSPR_LIBS="${LIBXUL_DIST}/lib/nspr${NSPR_VERSION}.lib ${LIBXUL_DIST}/lib/plc${NSPR_VERSION}.lib ${LIBXUL_DIST}/lib/plds${NSPR_VERSION}.lib "
+        fi
+    else
+        NSPR_CFLAGS=`"${LIBXUL_DIST}"/sdk/bin/nspr-config --prefix="${LIBXUL_DIST}" --includedir="${LIBXUL_DIST}/include/nspr" --cflags`
+        NSPR_LIBS=`"${LIBXUL_DIST}"/sdk/bin/nspr-config --prefix="${LIBXUL_DIST}" --libdir="${LIBXUL_DIST}"/lib --libs`
+    fi
+fi
 
 dnl set GRE_MILESTONE
 dnl ========================================================
@@ -9136,10 +9176,70 @@ if test -n "$_WRAP_MALLOC"; then
     _SUBDIR_CONFIG_ARGS="`echo $_SUBDIR_CONFIG_ARGS | sed -e 's/--enable-wrap-malloc *//'`"
 fi
 
-MOZ_SUBCONFIGURE_NSPR()
+if test -z "$MOZ_NATIVE_NSPR"; then
+    ac_configure_args="$_SUBDIR_CONFIG_ARGS --with-dist-prefix=$MOZ_BUILD_ROOT/dist --with-mozilla"
+    if test -z "$MOZ_DEBUG"; then
+        ac_configure_args="$ac_configure_args --disable-debug"
+    else
+        ac_configure_args="$ac_configure_args --enable-debug"
         if test -n "$MOZ_NO_DEBUG_RTL"; then
             ac_configure_args="$ac_configure_args --disable-debug-rtl"
         fi
+    fi
+    if test "$MOZ_OPTIMIZE" = "1"; then
+        ac_configure_args="$ac_configure_args --enable-optimize"
+    elif test -z "$MOZ_OPTIMIZE"; then
+        ac_configure_args="$ac_configure_args --disable-optimize"
+    fi
+    if test -n "$HAVE_64BIT_OS"; then
+        ac_configure_args="$ac_configure_args --enable-64bit"
+    fi
+    if test -n "$USE_ARM_KUSER"; then
+        ac_configure_args="$ac_configure_args --with-arm-kuser"
+    fi
+    # A configure script generated by autoconf 2.68 does not allow the cached
+    # values of "precious" variables such as CFLAGS and LDFLAGS to differ from
+    # the values passed to the configure script. Since we modify CFLAGS and
+    # LDFLAGS before passing them to NSPR's configure script, we cannot share
+    # config.cache with NSPR. As a result, we cannot pass AS, CC, CXX, etc. to
+    # NSPR via a shared config.cache file and must pass them to NSPR on the
+    # configure command line.
+    for var in AS CC CXX CPP LD AR RANLIB STRIP; do
+        ac_configure_args="$ac_configure_args $var='`eval echo \\${${var}}`'"
+    done
+    # A configure script generated by autoconf 2.68 warns if --host is
+    # specified but --build isn't. So we always pass --build to NSPR's
+    # configure script.
+    ac_configure_args="$ac_configure_args --build=$build"
+    ac_configure_args="$ac_configure_args $NSPR_CONFIGURE_ARGS"
+
+    # Save these, so we can mess with them for the subconfigure ..
+    _SAVE_CFLAGS="$CFLAGS"
+    _SAVE_CPPFLAGS="$CPPFLAGS"
+    _SAVE_LDFLAGS="$LDFLAGS"
+
+    if test -n "$MOZ_LINKER" -a "$ac_cv_func_dladdr" = no ; then
+      # dladdr is supported by the new linker, even when the system linker doesn't
+      # support it. Trick nspr into using dladdr when it's not supported.
+      export CPPFLAGS="-include $_topsrcdir/mozglue/linker/dladdr.h $CPPFLAGS"
+    fi
+    export LDFLAGS="$LDFLAGS $NSPR_LDFLAGS"
+    export CFLAGS="$CFLAGS $MOZ_FRAMEPTR_FLAGS"
+
+    # Use a separate cache file for NSPR since it uses autoconf 2.68.
+    _save_cache_file="$cache_file"
+    cache_file=$_objdir/nsprpub/config.cache
+
+    AC_OUTPUT_SUBDIRS(nsprpub)
+
+    # .. and restore them
+    cache_file="$_save_cache_file"
+    CFLAGS="$_SAVE_CFLAGS"
+    CPPFLAGS="$_SAVE_CPPFLAGS"
+    LDFLAGS="$_SAVE_LDFLAGS"
+
+    ac_configure_args="$_SUBDIR_CONFIG_ARGS"
+fi
 
 dnl ========================================================
 dnl = Setup a nice relatively clean build environment for
@@ -9162,14 +9262,10 @@ if test -n "$ENABLE_CLANG_PLUGIN"; then
     AC_OUTPUT_SUBDIRS(build/clang-plugin)
 fi
 
+
 # Run the SpiderMonkey 'configure' script.
 dist=$MOZ_BUILD_ROOT/dist
 ac_configure_args="$_SUBDIR_CONFIG_ARGS"
-
-# --with-system-nspr will have been converted into the relevant $NSPR_CFLAGS
-# and $NSPR_LIBS.
-ac_configure_args="`echo $ac_configure_args | sed -e 's/--with-system-nspr\S* *//'`"
-
 ac_configure_args="$ac_configure_args --enable-threadsafe"
 
 if test "$_INTL_API" = no; then
@@ -9192,7 +9288,7 @@ fi
 if test -z "$JSGC_GENERATIONAL" ; then
     ac_configure_args="$ac_configure_args --disable-gcgenerational"
 fi
-if test -n "$NSPR_CFLAGS" -o -n "$NSPR_LIBS"; then
+if test -z "$MOZ_NATIVE_NSPR"; then
     ac_configure_args="$ac_configure_args --with-nspr-cflags='$NSPR_CFLAGS'"
     ac_configure_args="$ac_configure_args --with-nspr-libs='$NSPR_LIBS'"
 fi
diff --git a/js/src/aclocal.m4 b/js/src/aclocal.m4
index 8560b0340a5..348d6d4c17b 100644
--- a/js/src/aclocal.m4
+++ b/js/src/aclocal.m4
@@ -12,7 +12,6 @@ builtin(include, ../../build/autoconf/ccache.m4)dnl
 builtin(include, ../../build/autoconf/wrapper.m4)dnl
 builtin(include, ../../build/autoconf/pkg.m4)dnl
 builtin(include, ../../build/autoconf/nspr.m4)dnl
-builtin(include, ../../build/autoconf/nspr-build.m4)dnl
 builtin(include, ../../build/autoconf/codeset.m4)dnl
 builtin(include, ../../build/autoconf/altoptions.m4)dnl
 builtin(include, ../../build/autoconf/mozprog.m4)dnl
diff --git a/js/src/configure.in b/js/src/configure.in
index 9ac8afc9138..98595d1e344 100644
--- a/js/src/configure.in
+++ b/js/src/configure.in
@@ -112,7 +112,7 @@ if test "$_conflict_files"; then
   exit 1
   break
 fi
-MOZ_BUILD_ROOT=`pwd -W 2>/dev/null || pwd`
+MOZ_BUILD_ROOT=`pwd`
 
 dnl Choose where to put the 'dist' directory.
 dnl ==============================================================
@@ -171,7 +171,6 @@ if test "$JS_STANDALONE" = no; then
   JS_STANDALONE=
 else
   JS_STANDALONE=1
-  LIBXUL_DIST="$MOZ_BUILD_ROOT/dist"
   AC_DEFINE(JS_STANDALONE)
 fi
 AC_SUBST(JS_STANDALONE)
@@ -1722,6 +1721,12 @@ ia64*-hpux*)
     no_x=yes
     AC_DEFINE(NO_X11)
 
+    case "$host" in
+    *-mingw*)
+        MOZ_BUILD_ROOT=`cd $MOZ_BUILD_ROOT && pwd -W`
+        ;;
+    esac
+
     case "$host" in
     *-mingw*)
         if test -z "$MOZ_TOOLS"; then
@@ -2753,6 +2758,34 @@ dnl =
 dnl ========================================================
 MOZ_ARG_HEADER(External Packages)
 
+dnl ========================================================
+dnl = Find the right NSPR to use.
+dnl ========================================================
+MOZ_ARG_WITH_BOOL(system-nspr,
+[  --with-system-nspr      Use an NSPR that is already built and installed.
+                          Use the 'nspr-config' script in the current path,
+                          or look for the script in the directories given with
+                          --with-nspr-exec-prefix or --with-nspr-prefix.
+                          (Those flags are only checked if you specify
+                          --with-system-nspr.)],
+    _USE_SYSTEM_NSPR=1 )
+
+MOZ_ARG_WITH_STRING(nspr-cflags,
+[  --with-nspr-cflags=FLAGS
+                          Pass FLAGS to CC when building code that uses NSPR.
+                          Use this when there's no accurate nspr-config
+                          script available.  This is the case when building
+                          SpiderMonkey as part of the Mozilla tree: the
+                          top-level configure script computes NSPR flags
+                          that accomodate the quirks of that environment.],
+    NSPR_CFLAGS=$withval)
+MOZ_ARG_WITH_STRING(nspr-libs,
+[  --with-nspr-libs=LIBS   Pass LIBS to LD when linking code that uses NSPR.
+                          See --with-nspr-cflags for more details.],
+    NSPR_LIBS=$withval)
+AC_SUBST(NSPR_CFLAGS)
+AC_SUBST(NSPR_LIBS)
+
 JS_THREADSAFE=1
 MOZ_ARG_DISABLE_BOOL(threadsafe,
 [  --disable-threadsafe    Disable support for multiple threads.],
@@ -2764,7 +2797,64 @@ fi
 JS_THREADSAFE_CONFIGURED=$JS_THREADSAFE
 AC_SUBST(JS_THREADSAFE_CONFIGURED)
 
-MOZ_CONFIG_NSPR(js)
+if test "$_USE_SYSTEM_NSPR" || (test "$NSPR_CFLAGS" -o "$NSPR_LIBS"); then
+  _HAS_NSPR=1
+fi
+
+case "$target" in
+  *linux*|*darwin*|*dragonfly*|*freebsd*|*netbsd*|*openbsd*)
+    if test -z "$_HAS_NSPR" && test "$JS_THREADSAFE"; then
+      JS_POSIX_NSPR_DEFAULT=1
+    fi
+    ;;
+esac
+
+MOZ_ARG_ENABLE_BOOL(posix-nspr-emulation,
+[  --enable-posix-nspr-emulation
+                          Enable emulation of NSPR for POSIX systems],
+    JS_POSIX_NSPR=1,
+    JS_POSIX_NSPR=,
+    JS_POSIX_NSPR="$JS_POSIX_NSPR_DEFAULT" )
+if test -n "$JS_POSIX_NSPR"; then
+    AC_DEFINE(JS_POSIX_NSPR)
+fi
+
+AC_SUBST(JS_POSIX_NSPR)
+
+dnl Pass either --with-system-nspr or (--with-nspr-cflags and
+dnl --with-nspr-libs), but not both.
+if test "$_USE_SYSTEM_NSPR" && (test "$NSPR_CFLAGS" -o "$NSPR_LIBS"); then
+    AC_MSG_ERROR([--with-system-nspr and --with-nspr-libs/cflags are mutually exclusive.
+See 'configure --help'.])
+fi
+
+dnl Can't use --enable-posix-nspr-emulation if compiling with NSPR.
+if test "$_HAS_NSPR" && test "$JS_POSIX_NSPR"; then
+    AC_MSG_ERROR([--enable-posix-nspr-emulation is mututally exclusive with --with-system-nspr
+and --with-nspr-libs/cflags. See 'configure --help'.])
+fi
+
+if test -n "$_USE_SYSTEM_NSPR"; then
+    MOZ_NATIVE_NSPR=
+    AM_PATH_NSPR($NSPR_MINVER, [MOZ_NATIVE_NSPR=1], [AC_MSG_ERROR([your don't have NSPR installed or your version is too old])])
+fi
+
+NSPR_PKGCONF_CHECK="nspr"
+if test -n "$MOZ_NATIVE_NSPR"; then
+    # piggy back on $MOZ_NATIVE_NSPR to set a variable for the nspr check for js.pc
+    NSPR_PKGCONF_CHECK="nspr >= $NSPR_MINVER"
+
+    _SAVE_CFLAGS=$CFLAGS
+    CFLAGS="$CFLAGS $NSPR_CFLAGS"
+    AC_TRY_COMPILE([#include "prlog.h"],
+                [#ifndef PR_STATIC_ASSERT
+                 #error PR_STATIC_ASSERT not defined
+                 #endif],
+                [MOZ_NATIVE_NSPR=1],
+                AC_MSG_ERROR([system NSPR does not support PR_STATIC_ASSERT]))
+    CFLAGS=$_SAVE_CFLAGS
+fi
+AC_SUBST(NSPR_PKGCONF_CHECK)
 
 dnl ========================================================
 dnl system zlib Support
@@ -3951,6 +4041,7 @@ AC_SUBST(ac_configure_args)
 
 AC_SUBST(TOOLCHAIN_PREFIX)
 
+
 if test -n "$JS_STANDALONE"; then
 MOZ_APP_NAME="mozjs"
 MOZ_APP_VERSION="$MOZILLA_SYMBOLVERSION"
@@ -3968,9 +4059,6 @@ AC_SUBST(JS_LIBRARY_NAME)
 AC_SUBST(JS_CONFIG_MOZ_JS_LIBS)
 AC_SUBST(JS_CONFIG_LIBS)
 
-if test -n "$MOZ_BUILD_NSPR"; then
-    MOZ_SUBCONFIGURE_NSPR()
-fi
 MOZ_SUBCONFIGURE_FFI()
 
 dnl Spit out some output
diff --git a/moz.build b/moz.build
index 93042be6a28..8380145017f 100644
--- a/moz.build
+++ b/moz.build
@@ -39,15 +39,16 @@ if not CONFIG['LIBXUL_SDK']:
 if not CONFIG['JS_STANDALONE']:
     add_tier_dir('precompile', 'xpcom/xpidl')
 
-if CONFIG['COMPILE_ENVIRONMENT'] and not CONFIG['LIBXUL_SDK']:
-    if CONFIG['MOZ_BUILD_NSPR']:
-        add_tier_dir('nspr', 'config/nspr')
+    if CONFIG['COMPILE_ENVIRONMENT'] and not CONFIG['LIBXUL_SDK']:
+        if not CONFIG['MOZ_NATIVE_NSPR']:
+            add_tier_dir('nspr', 'config/nspr')
 
-    if not CONFIG['JS_STANDALONE']:
         add_tier_dir('external', 'config/external')
+
         if not CONFIG['MOZ_NATIVE_NSS']:
              add_tier_dir('nss', 'security/build')
 
+if CONFIG['COMPILE_ENVIRONMENT'] and not CONFIG['LIBXUL_SDK']:
     if CONFIG['BUILD_CTYPES'] and not CONFIG['MOZ_NATIVE_FFI']:
         add_tier_dir('js', ['js/src/ctypes/libffi'], static=True)
     add_tier_dir('js', ['intl/icu'], static=True)