--- a/build/ConfigStatus.py
+++ b/build/ConfigStatus.py
@@ -119,17 +119,18 @@ class ConfigEnvironment(object):
          preprocessor command lines. The order in which defines were given
          when creating the ConfigEnvironment is preserved.
        - ALLDEFINES contains the defines in the form #define NAME VALUE, in
          sorted order, for use in config files, for an automatic listing of
          defines.
 
     ConfigEnvironment expects a "top_srcdir" subst to be set with the top
     source directory, in msys format on windows. It is used to derive a
-    "srcdir" subst when treating config files.
+    "srcdir" subst when treating config files. It can either be an absolute
+    path or a path relative to the topobjdir.
     '''
 
     def __init__(self, topobjdir = '.', topsrcdir = '.',
                  defines = [], non_global_defines = [], substs = []):
         self.defines = dict(defines)
         self.substs = dict(substs)
         self.topsrcdir = topsrcdir
         self.topobjdir = topobjdir
@@ -142,22 +143,32 @@ class ConfigEnvironment(object):
         using / as a path separator.
         '''
         assert(isinstance(file, basestring))
         dir = posixpath.dirname(relpath(file, self.topobjdir).replace(os.sep, '/'))
         if dir:
             return dir
         return '.'
 
+    def get_top_srcdir(self, file):
+        '''Returns a normalized top_srcdir for the given file: if
+        substs['top_srcdir'] is a relative path, it is relative to the
+        topobjdir. Adjust it to be relative to the file path.'''
+        top_srcdir = self.substs['top_srcdir']
+        if posixpath.isabs(top_srcdir):
+            return top_srcdir
+        return posixpath.normpath(posixpath.join(self.get_depth(file), top_srcdir))
+
     def get_file_srcdir(self, file):
         '''Returns the srcdir for the given file, where srcdir is in msys
         format on windows, thus derived from top_srcdir.
         '''
         dir = self.get_relative_srcdir(file)
-        return posixpath.normpath(posixpath.join(self.substs['top_srcdir'], dir))
+        top_srcdir = self.get_top_srcdir(file)
+        return posixpath.normpath(posixpath.join(top_srcdir, dir))
 
     def get_depth(self, file):
         '''Returns the DEPTH for the given file, that is, the path to the
         object directory relative to the directory containing the given file.
         Always uses / as a path separator.
         '''
         return relpath(self.topobjdir, os.path.dirname(file)).replace(os.sep, '/')
 
@@ -176,16 +187,17 @@ class ConfigEnvironment(object):
         Additional substs are defined according to the file being treated:
             "srcdir" for its the path to its source directory
             "relativesrcdir" for its source directory relative to the top
             "DEPTH" for the path to the top object directory
         '''
         input = self.get_input(path)
         pp = Preprocessor()
         pp.context.update(self.substs)
+        pp.context.update(top_srcdir = self.get_top_srcdir(path))
         pp.context.update(srcdir = self.get_file_srcdir(path))
         pp.context.update(relativesrcdir = self.get_relative_srcdir(path))
         pp.context.update(DEPTH = self.get_depth(path))
         pp.do_filter('attemptSubstitution')
         pp.setMarker(None)
         with FileAvoidWrite(path) as pp.out:
             pp.do_include(input)
 

--- a/build/tests/unit-ConfigStatus.py
+++ b/build/tests/unit-ConfigStatus.py
@@ -1,22 +1,25 @@
 from __future__ import with_statement
-import os
+import os, posixpath
 from StringIO import StringIO
 import unittest
 from mozunit import main, MockedOpen
 import ConfigStatus
 from ConfigStatus import FileAvoidWrite
 
 class ConfigEnvironment(ConfigStatus.ConfigEnvironment):
     def __init__(self, **args):
         ConfigStatus.ConfigEnvironment.__init__(self, **args)
         # Be helpful to unit tests
         if not 'top_srcdir' in self.substs:
-            self.substs['top_srcdir'] = self.topsrcdir.replace(os.sep, '/')
+            if os.path.isabs(self.topsrcdir):
+                self.substs['top_srcdir'] = self.topsrcdir.replace(os.sep, '/')
+            else:
+                self.substs['top_srcdir'] = ConfigStatus.relpath(self.topsrcdir, self.topobjdir).replace(os.sep, '/')
 
 class TestFileAvoidWrite(unittest.TestCase):
     def test_file_avoid_write(self):
         '''Test the FileAvoidWrite class
         '''
         with MockedOpen({'file': 'content'}):
             # Overwriting an existing file replaces its content
             with FileAvoidWrite('file') as file:
@@ -135,34 +138,44 @@ class TestPathsLocalBuildDir(TestPaths):
         self.assertEqual(env.get_relative_srcdir('dir/file'), 'dir')
         self.assertEqual(env.get_relative_srcdir('deeply/nested/path/to/file'), 'deeply/nested/path/to')
         self.assertEqual(env.get_depth('file'), '.')
         self.assertEqual(env.get_depth('dir/file'), '..')
         self.assertEqual(env.get_depth('deeply/nested/path/to/file'), '../../../..')
         return env
 
     def test_paths_local_build_local_src(self):
+        # topsrcdir = . ; topobjdir = .
         env = self.get_env('.')
         self.assertEqual(env.get_input('file'), 'file.in')
         self.assertEqual(env.get_input('dir/file'), os.path.join('dir', 'file.in'))
+        self.assertEqual(env.get_top_srcdir('file'), '.')
+        self.assertEqual(env.get_top_srcdir('dir/file'), '..')
         self.assertEqual(env.get_file_srcdir('file'), '.')
-        self.assertEqual(env.get_file_srcdir('dir/file'), 'dir')
+        self.assertEqual(env.get_file_srcdir('dir/file'), '../dir')
 
     def test_paths_local_build_parent_src(self):
+        # topsrcdir = .. ; topobjdir = .
         env = self.get_env('..')
         self.assertEqual(env.get_input('file'), os.path.join('..', 'file.in'))
         self.assertEqual(env.get_input('dir/file'), os.path.join('..', 'dir', 'file.in'))
+        self.assertEqual(env.get_top_srcdir('file'), '..')
+        self.assertEqual(env.get_top_srcdir('dir/file'), '../..')
         self.assertEqual(env.get_file_srcdir('file'), '..')
-        self.assertEqual(env.get_file_srcdir('dir/file'), '../dir')
+        self.assertEqual(env.get_file_srcdir('dir/file'), '../../dir')
 
     def test_paths_local_build_absolute_src(self):
+        # topsrcdir = /absolute ; topobjdir = /absolute
         env = self.get_env(self.absolute)
         self.assertEqual(env.get_input('file'), os.path.join(self.absolute, 'file.in'))
         self.assertEqual(env.get_input('dir/file'), os.path.join(self.absolute, 'dir', 'file.in'))
         self.assertEqual(env.get_input('%s/file' % self.dir), os.path.join(self.absolute, self.dir, 'file.in'))
+        self.assertEqual(env.get_top_srcdir('file'), '/absolute')
+        self.assertEqual(env.get_top_srcdir('dir/file'), '/absolute')
+        self.assertEqual(env.get_top_srcdir('%s/file' % dir), '/absolute')
         self.assertEqual(env.get_file_srcdir('file'), '/absolute')
         self.assertEqual(env.get_file_srcdir('dir/file'), '/absolute/dir')
         self.assertEqual(env.get_file_srcdir('%s/file' % dir), '/absolute/%s' % dir)
 
 class TestPathsParentBuildDir(TestPaths):
     def get_env(self, topsrcdir):
         env = ConfigEnvironment(topsrcdir = topsrcdir, topobjdir = '..')
         self.assertEqual(env.get_relative_srcdir('..'), '.')
@@ -171,78 +184,102 @@ class TestPathsParentBuildDir(TestPaths)
         self.assertEqual(env.get_relative_srcdir('deeply/nested/path/to/file'), '%s/deeply/nested/path/to' % self.dir)
         self.assertEqual(env.get_depth('../file'), '.')
         self.assertEqual(env.get_depth('file'), '..')
         self.assertEqual(env.get_depth('dir/file'), '../..')
         self.assertEqual(env.get_depth('deeply/nested/path/to/file'), '../../../../..')
         return env
 
     def test_paths_parent_build_parent_src(self):
+        # topsrcdir = .. ; topobjdir = ..
         env = self.get_env('..')
         self.assertEqual(env.get_input('../file'), os.path.join('..', 'file.in'))
         self.assertEqual(env.get_input('file'), os.path.join('..', self.dir, 'file.in'))
         self.assertEqual(env.get_input('dir/file'), os.path.join('..', self.dir, 'dir', 'file.in'))
-        self.assertEqual(env.get_file_srcdir('../file'), '..')
+        self.assertEqual(env.get_top_srcdir('../file'), '.')
+        self.assertEqual(env.get_top_srcdir('file'), '..')
+        self.assertEqual(env.get_top_srcdir('dir/file'), '../..')
+        self.assertEqual(env.get_file_srcdir('../file'), '.')
         self.assertEqual(env.get_file_srcdir('file'), '../%s' % self.dir)
-        self.assertEqual(env.get_file_srcdir('dir/file'), '../%s/dir' % self.dir)
+        self.assertEqual(env.get_file_srcdir('dir/file'), '../../%s/dir' % self.dir)
 
     def test_paths_parent_build_ancestor_src(self):
+        # topsrcdir = ../.. ; topobjdir = ..
         env = self.get_env('../..')
         self.assertEqual(env.get_input('../file'), os.path.join('..', '..', 'file.in'))
         self.assertEqual(env.get_input('file'), os.path.join('..', '..', self.dir, 'file.in'))
         self.assertEqual(env.get_input('dir/file'), os.path.join('..', '..', self.dir, 'dir', 'file.in'))
-        self.assertEqual(env.get_file_srcdir('../file'), '../..')
+        self.assertEqual(env.get_top_srcdir('../file'), '..')
+        self.assertEqual(env.get_top_srcdir('file'), '../..')
+        self.assertEqual(env.get_top_srcdir('dir/file'), '../../..')
+        self.assertEqual(env.get_file_srcdir('../file'), '..')
         self.assertEqual(env.get_file_srcdir('file'), '../../%s' % self.dir)
-        self.assertEqual(env.get_file_srcdir('dir/file'), '../../%s/dir' % self.dir)
+        self.assertEqual(env.get_file_srcdir('dir/file'), '../../../%s/dir' % self.dir)
 
     def test_paths_parent_build_absolute_src(self):
+        # topsrcdir = /absolute ; topobjdir = ..
         env = self.get_env(self.absolute)
         self.assertEqual(env.get_input('../file'), os.path.join(self.absolute, 'file.in'))
         self.assertEqual(env.get_input('file'), os.path.join(self.absolute, self.dir, 'file.in'))
         self.assertEqual(env.get_input('dir/file'), os.path.join(self.absolute, self.dir, 'dir', 'file.in'))
+        self.assertEqual(env.get_top_srcdir('../file'), '/absolute')
+        self.assertEqual(env.get_top_srcdir('file'), '/absolute')
+        self.assertEqual(env.get_top_srcdir('dir/file'), '/absolute')
         self.assertEqual(env.get_file_srcdir('../file'), '/absolute')
         self.assertEqual(env.get_file_srcdir('file'), '/absolute/%s' % self.dir)
         self.assertEqual(env.get_file_srcdir('dir/file'), '/absolute/%s/dir' % self.dir)
 
 class TestPathsRelativeBuild(TestPaths):
     def get_env(self, topsrcdir):
         env = ConfigEnvironment(topsrcdir = topsrcdir, topobjdir = 'relative')
         self.assertEqual(env.get_relative_srcdir('relative/file'), '.')
         self.assertEqual(env.get_relative_srcdir('relative/dir/file'), 'dir')
         self.assertEqual(env.get_relative_srcdir('relative/deeply/nested/path/to/file'), 'deeply/nested/path/to')
         self.assertEqual(env.get_depth('relative/file'), '.')
         self.assertEqual(env.get_depth('relative/dir/file'), '..')
         self.assertEqual(env.get_depth('relative/deeply/nested/path/to/file'), '../../../..')
         return env
 
     def test_paths_relative_build_relative_src(self):
+        # topsrcdir = relative ; topobjdir = relative
         env = self.get_env('relative')
         self.assertEqual(env.get_input('relative/file'), os.path.join('relative', 'file.in'))
         self.assertEqual(env.get_input('relative/dir/file'), os.path.join('relative', 'dir', 'file.in'))
-        self.assertEqual(env.get_file_srcdir('relative/file'), 'relative')
-        self.assertEqual(env.get_file_srcdir('relative/dir/file'), 'relative/dir')
+        self.assertEqual(env.get_top_srcdir('relative/file'), '.')
+        self.assertEqual(env.get_top_srcdir('relative/dir/file'), '..')
+        self.assertEqual(env.get_file_srcdir('relative/file'), '.')
+        self.assertEqual(env.get_file_srcdir('relative/dir/file'), '../dir')
 
     def test_paths_relative_build_local_src(self):
+        # topsrcdir = . ; topobjdir = relative
         env = self.get_env('.')
         self.assertEqual(env.get_input('relative/file'), 'file.in')
         self.assertEqual(env.get_input('relative/dir/file'), os.path.join('dir', 'file.in'))
-        self.assertEqual(env.get_file_srcdir('relative/file'), '.')
-        self.assertEqual(env.get_file_srcdir('relative/dir/file'), 'dir')
+        self.assertEqual(env.get_top_srcdir('relative/file'), '..')
+        self.assertEqual(env.get_top_srcdir('relative/dir/file'), '../..')
+        self.assertEqual(env.get_file_srcdir('relative/file'), '..')
+        self.assertEqual(env.get_file_srcdir('relative/dir/file'), '../../dir')
 
     def test_paths_relative_build_parent_src(self):
+        # topsrcdir = .. ; topobjdir = relative
         env = self.get_env('..')
         self.assertEqual(env.get_input('relative/file'), os.path.join('..', 'file.in'))
         self.assertEqual(env.get_input('relative/dir/file'), os.path.join('..', 'dir', 'file.in'))
-        self.assertEqual(env.get_file_srcdir('relative/file'), '..')
-        self.assertEqual(env.get_file_srcdir('relative/dir/file'), '../dir')
+        self.assertEqual(env.get_top_srcdir('relative/file'), '../..')
+        self.assertEqual(env.get_top_srcdir('relative/dir/file'), '../../..')
+        self.assertEqual(env.get_file_srcdir('relative/file'), '../..')
+        self.assertEqual(env.get_file_srcdir('relative/dir/file'), '../../../dir')
 
     def test_paths_relative_build_absolute_src(self):
+        # topsrcdir = /absolute ; topobjdir = relative
         env = self.get_env(self.absolute)
         self.assertEqual(env.get_input('relative/file'), os.path.join(self.absolute, 'file.in'))
         self.assertEqual(env.get_input('relative/dir/file'), os.path.join(self.absolute, 'dir', 'file.in'))
+        self.assertEqual(env.get_top_srcdir('relative/file'), '/absolute')
+        self.assertEqual(env.get_top_srcdir('relative/dir/file'), '/absolute')
         self.assertEqual(env.get_file_srcdir('relative/file'), '/absolute')
         self.assertEqual(env.get_file_srcdir('relative/dir/file'), '/absolute/dir')
 
 class TestPathsAbsoluteBuild(unittest.TestCase):
     def setUp(self):
         self.absolute_build = os.path.normpath('/absolute/build')
 
     def get_env(self, topsrcdir):
@@ -251,32 +288,41 @@ class TestPathsAbsoluteBuild(unittest.Te
         self.assertEqual(env.get_relative_srcdir('/absolute/build/dir/file'), 'dir')
         self.assertEqual(env.get_relative_srcdir('/absolute/build/deeply/nested/path/to/file'), 'deeply/nested/path/to')
         self.assertEqual(env.get_depth('/absolute/build/file'), '.')
         self.assertEqual(env.get_depth('/absolute/build/dir/file'), '..')
         self.assertEqual(env.get_depth('/absolute/build/deeply/nested/path/to/file'), '../../../..')
         return env
 
     def test_paths_absolute_build_same_src(self):
+        # topsrcdir = /absolute/build ; topobjdir = /absolute/build
         env = self.get_env(self.absolute_build)
         self.assertEqual(env.get_input('/absolute/build/file'), os.path.join(self.absolute_build, 'file.in'))
         self.assertEqual(env.get_input('/absolute/build/dir/file'), os.path.join(self.absolute_build, 'dir', 'file.in'))
+        self.assertEqual(env.get_top_srcdir('/absolute/build/file'), '/absolute/build')
+        self.assertEqual(env.get_top_srcdir('/absolute/build/dir/file'), '/absolute/build')
         self.assertEqual(env.get_file_srcdir('/absolute/build/file'), '/absolute/build')
         self.assertEqual(env.get_file_srcdir('/absolute/build/dir/file'), '/absolute/build/dir')
 
     def test_paths_absolute_build_ancestor_src(self):
+        # topsrcdir = /absolute ; topobjdir = /absolute/build
         absolute = os.path.dirname(self.absolute_build)
         env = self.get_env(absolute)
         self.assertEqual(env.get_input('/absolute/build/file'), os.path.join(absolute, 'file.in'))
         self.assertEqual(env.get_input('/absolute/build/dir/file'), os.path.join(absolute, 'dir', 'file.in'))
+        self.assertEqual(env.get_top_srcdir('/absolute/build/file'), '/absolute')
+        self.assertEqual(env.get_top_srcdir('/absolute/build/dir/file'), '/absolute')
         self.assertEqual(env.get_file_srcdir('/absolute/build/file'), '/absolute')
         self.assertEqual(env.get_file_srcdir('/absolute/build/dir/file'), '/absolute/dir')
 
     def test_paths_absolute_build_different_src(self):
+        # topsrcdir = /some/path ; topobjdir = /absolute/build
         absolute = os.path.normpath('/some/path')
         env = self.get_env(absolute)
         self.assertEqual(env.get_input('/absolute/build/file'), os.path.join(absolute, 'file.in'))
         self.assertEqual(env.get_input('/absolute/build/dir/file'), os.path.join(absolute, 'dir', 'file.in'))
+        self.assertEqual(env.get_top_srcdir('/absolute/build/file'), '/some/path')
+        self.assertEqual(env.get_top_srcdir('/absolute/build/dir/file'), '/some/path')
         self.assertEqual(env.get_file_srcdir('/absolute/build/file'), '/some/path')
         self.assertEqual(env.get_file_srcdir('/absolute/build/dir/file'), '/some/path/dir')
 
 if __name__ == "__main__":
     main()

--- a/js/src/build/ConfigStatus.py
+++ b/js/src/build/ConfigStatus.py
@@ -119,17 +119,18 @@ class ConfigEnvironment(object):
          preprocessor command lines. The order in which defines were given
          when creating the ConfigEnvironment is preserved.
        - ALLDEFINES contains the defines in the form #define NAME VALUE, in
          sorted order, for use in config files, for an automatic listing of
          defines.
 
     ConfigEnvironment expects a "top_srcdir" subst to be set with the top
     source directory, in msys format on windows. It is used to derive a
-    "srcdir" subst when treating config files.
+    "srcdir" subst when treating config files. It can either be an absolute
+    path or a path relative to the topobjdir.
     '''
 
     def __init__(self, topobjdir = '.', topsrcdir = '.',
                  defines = [], non_global_defines = [], substs = []):
         self.defines = dict(defines)
         self.substs = dict(substs)
         self.topsrcdir = topsrcdir
         self.topobjdir = topobjdir
@@ -142,22 +143,32 @@ class ConfigEnvironment(object):
         using / as a path separator.
         '''
         assert(isinstance(file, basestring))
         dir = posixpath.dirname(relpath(file, self.topobjdir).replace(os.sep, '/'))
         if dir:
             return dir
         return '.'
 
+    def get_top_srcdir(self, file):
+        '''Returns a normalized top_srcdir for the given file: if
+        substs['top_srcdir'] is a relative path, it is relative to the
+        topobjdir. Adjust it to be relative to the file path.'''
+        top_srcdir = self.substs['top_srcdir']
+        if posixpath.isabs(top_srcdir):
+            return top_srcdir
+        return posixpath.normpath(posixpath.join(self.get_depth(file), top_srcdir))
+
     def get_file_srcdir(self, file):
         '''Returns the srcdir for the given file, where srcdir is in msys
         format on windows, thus derived from top_srcdir.
         '''
         dir = self.get_relative_srcdir(file)
-        return posixpath.normpath(posixpath.join(self.substs['top_srcdir'], dir))
+        top_srcdir = self.get_top_srcdir(file)
+        return posixpath.normpath(posixpath.join(top_srcdir, dir))
 
     def get_depth(self, file):
         '''Returns the DEPTH for the given file, that is, the path to the
         object directory relative to the directory containing the given file.
         Always uses / as a path separator.
         '''
         return relpath(self.topobjdir, os.path.dirname(file)).replace(os.sep, '/')
 
@@ -176,16 +187,17 @@ class ConfigEnvironment(object):
         Additional substs are defined according to the file being treated:
             "srcdir" for its the path to its source directory
             "relativesrcdir" for its source directory relative to the top
             "DEPTH" for the path to the top object directory
         '''
         input = self.get_input(path)
         pp = Preprocessor()
         pp.context.update(self.substs)
+        pp.context.update(top_srcdir = self.get_top_srcdir(path))
         pp.context.update(srcdir = self.get_file_srcdir(path))
         pp.context.update(relativesrcdir = self.get_relative_srcdir(path))
         pp.context.update(DEPTH = self.get_depth(path))
         pp.do_filter('attemptSubstitution')
         pp.setMarker(None)
         with FileAvoidWrite(path) as pp.out:
             pp.do_include(input)