--- a/python/mozbuild/mozbuild/backend/recursivemake.py
+++ b/python/mozbuild/mozbuild/backend/recursivemake.py
@@ -504,26 +504,30 @@ class RecursiveMakeBackend(CommonBackend
         elif isinstance(obj, HostDefines):
             self._process_defines(obj, backend_file, which='HOST_DEFINES')
         elif isinstance(obj, Defines):
             self._process_defines(obj, backend_file)
 
         elif isinstance(obj, GeneratedFile):
             tier = 'misc' if any(isinstance(f, ObjDirPath) for f in obj.inputs) else 'export'
             self._no_skip[tier].add(backend_file.relobjdir)
-            dep_file = "%s.pp" % obj.output
-            backend_file.write('%s:: %s\n' % (tier, obj.output))
-            backend_file.write('GARBAGE += %s\n' % obj.output)
+            first_output = obj.outputs[0]
+            dep_file = "%s.pp" % first_output
+            backend_file.write('%s:: %s\n' % (tier, first_output))
+            for output in obj.outputs:
+                if output != first_output:
+                    backend_file.write('%s: %s ;\n' % (output, first_output))
+                backend_file.write('GARBAGE += %s\n' % output)
             backend_file.write('EXTRA_MDDEPEND_FILES += %s\n' % dep_file)
             if obj.script:
                 backend_file.write("""{output}: {script}{inputs}{backend}
 \t$(REPORT_BUILD)
 \t$(call py_action,file_generate,{script} {method} {output} $(MDDEPDIR)/{dep_file}{inputs}{flags})
 
-""".format(output=obj.output,
+""".format(output=first_output,
            dep_file=dep_file,
            inputs=' ' + ' '.join([f.full_path for f in obj.inputs]) if obj.inputs else '',
            flags=' ' + ' '.join(obj.flags) if obj.flags else '',
            backend=' backend.mk' if obj.flags else '',
            script=obj.script,
            method=obj.method))
 
         elif isinstance(obj, JARManifest):

--- a/python/mozbuild/mozbuild/frontend/data.py
+++ b/python/mozbuild/mozbuild/frontend/data.py
@@ -907,26 +907,26 @@ class SdkFiles(FinalTargetFiles):
 
 
 class GeneratedFile(ContextDerived):
     """Represents a generated file."""
 
     __slots__ = (
         'script',
         'method',
-        'output',
+        'outputs',
         'inputs',
         'flags',
     )
 
-    def __init__(self, context, script, method, output, inputs, flags=()):
+    def __init__(self, context, script, method, outputs, inputs, flags=()):
         ContextDerived.__init__(self, context)
         self.script = script
         self.method = method
-        self.output = output
+        self.outputs = outputs if isinstance(outputs, tuple) else (outputs,)
         self.inputs = inputs
         self.flags = flags
 
 
 class ClassPathEntry(object):
     """Represents a classpathentry in an Android Eclipse project."""
 
     __slots__ = (

--- a/python/mozbuild/mozbuild/frontend/emitter.py
+++ b/python/mozbuild/mozbuild/frontend/emitter.py
@@ -780,17 +780,18 @@ class TreeMetadataEmitter(LoggingMixin):
             if (context.config.substs.get('MOZ_DEBUG') and
                     not context.config.substs.get('MOZ_NO_DEBUG_RTL')):
                 rtl_flag += 'd'
             # Use a list, like MOZBUILD_*FLAGS variables
             passthru.variables['RTL_FLAGS'] = [rtl_flag]
 
         generated_files = set()
         for obj in self._process_generated_files(context):
-            generated_files.add(obj.output)
+            for f in obj.outputs:
+                generated_files.add(f)
             yield obj
 
         for path in context['CONFIGURE_SUBST_FILES']:
             sub = self._create_substitution(ConfigFileSubstitution, context,
                 path)
             generated_files.add(str(sub.relpath))
             yield sub
 
@@ -1000,17 +1001,17 @@ class TreeMetadataEmitter(LoggingMixin):
                                 [Path(context, path + '.in')])
 
         generated_files = context.get('GENERATED_FILES')
         if not generated_files:
             return
 
         for f in generated_files:
             flags = generated_files[f]
-            output = f
+            outputs = f
             inputs = []
             if flags.script:
                 method = "main"
                 script = SourcePath(context, flags.script).full_path
 
                 # Deal with cases like "C:\\path\\to\\script.py:function".
                 if '.py:' in script:
                     script, method = script.rsplit('.py:', 1)
@@ -1031,17 +1032,17 @@ class TreeMetadataEmitter(LoggingMixin):
                             not os.path.exists(p.full_path)):
                         raise SandboxValidationError(
                             'Input for generating %s does not exist: %s'
                             % (f, p.full_path), context)
                     inputs.append(p)
             else:
                 script = None
                 method = None
-            yield GeneratedFile(context, script, method, output, inputs)
+            yield GeneratedFile(context, script, method, outputs, inputs)
 
     def _process_test_manifests(self, context):
         for prefix, info in TEST_MANIFESTS.items():
             for path, manifest in context.get('%s_MANIFESTS' % prefix, []):
                 for obj in self._process_test_manifest(context, info, path, manifest):
                     yield obj
 
         for flavor in REFTEST_FLAVORS:

--- a/python/mozbuild/mozbuild/test/frontend/data/generated-files/moz.build
+++ b/python/mozbuild/mozbuild/test/frontend/data/generated-files/moz.build
@@ -1,5 +1,5 @@
 # -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
 # Any copyright is dedicated to the Public Domain.
 # http://creativecommons.org/publicdomain/zero/1.0/
 
-GENERATED_FILES += [ 'bar.c', 'foo.c' ]
+GENERATED_FILES += [ 'bar.c', 'foo.c', ('xpidllex.py', 'xpidlyacc.py'), ]

--- a/python/mozbuild/mozbuild/test/frontend/test_emitter.py
+++ b/python/mozbuild/mozbuild/test/frontend/test_emitter.py
@@ -224,51 +224,52 @@ class TestEmitterBasic(unittest.TestCase
                           'AS_DASH_C_FLAG': ''})
         self.maxDiff = maxDiff
 
 
     def test_generated_files(self):
         reader = self.reader('generated-files')
         objs = self.read_topsrcdir(reader)
 
-        self.assertEqual(len(objs), 2)
+        self.assertEqual(len(objs), 3)
         for o in objs:
             self.assertIsInstance(o, GeneratedFile)
 
-        expected = ['bar.c', 'foo.c']
-        for o, expected_filename in zip(objs, expected):
-            self.assertEqual(o.output, expected_filename)
+        expected = ['bar.c', 'foo.c', ('xpidllex.py', 'xpidlyacc.py'), ]
+        for o, f in zip(objs, expected):
+            expected_filename = f if isinstance(f, tuple) else (f,)
+            self.assertEqual(o.outputs, expected_filename)
             self.assertEqual(o.script, None)
             self.assertEqual(o.method, None)
             self.assertEqual(o.inputs, [])
 
     def test_generated_files_method_names(self):
         reader = self.reader('generated-files-method-names')
         objs = self.read_topsrcdir(reader)
 
         self.assertEqual(len(objs), 2)
         for o in objs:
             self.assertIsInstance(o, GeneratedFile)
 
         expected = ['bar.c', 'foo.c']
         expected_method_names = ['make_bar', 'main']
         for o, expected_filename, expected_method in zip(objs, expected, expected_method_names):
-            self.assertEqual(o.output, expected_filename)
+            self.assertEqual(o.outputs, (expected_filename,))
             self.assertEqual(o.method, expected_method)
             self.assertEqual(o.inputs, [])
 
     def test_generated_files_absolute_script(self):
         reader = self.reader('generated-files-absolute-script')
         objs = self.read_topsrcdir(reader)
 
         self.assertEqual(len(objs), 1)
 
         o = objs[0]
         self.assertIsInstance(o, GeneratedFile)
-        self.assertEqual(o.output, 'bar.c')
+        self.assertEqual(o.outputs, ('bar.c',))
         self.assertRegexpMatches(o.script, 'script.py$')
         self.assertEqual(o.method, 'make_bar')
         self.assertEqual(o.inputs, [])
 
     def test_generated_files_no_script(self):
         reader = self.reader('generated-files-no-script')
         with self.assertRaisesRegexp(SandboxValidationError,
             'Script for generating bar.c does not exist'):

--- a/python/mozbuild/mozbuild/util.py
+++ b/python/mozbuild/mozbuild/util.py
@@ -411,17 +411,21 @@ class UnsortedError(Exception):
 
 
 class StrictOrderingOnAppendListMixin(object):
     @staticmethod
     def ensure_sorted(l):
         if isinstance(l, StrictOrderingOnAppendList):
             return
 
-        srtd = sorted(l, key=lambda x: x.lower())
+        def _first_element(e):
+            # If the list entry is a tuple, we sort based on the first element
+            # in the tuple.
+            return e[0] if isinstance(e, tuple) else e
+        srtd = sorted(l, key=lambda x: _first_element(x).lower())
 
         if srtd != l:
             raise UnsortedError(srtd, l)
 
     def __init__(self, iterable=None, **kwargs):
         if iterable is None:
             iterable = []
 

--- a/xpcom/idl-parser/xpidl/moz.build
+++ b/xpcom/idl-parser/xpidl/moz.build
@@ -4,21 +4,20 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 PYTHON_UNIT_TESTS += [
     'runtests.py',
 ]
 
 GENERATED_FILES += [
-    'xpidllex.py',
-    'xpidlyacc.py',
+    ('xpidllex.py', 'xpidlyacc.py'),
 ]
 
-GENERATED_FILES['xpidllex.py'].script = 'header.py:main'
+GENERATED_FILES[('xpidllex.py', 'xpidlyacc.py')].script = 'header.py:main'
 
 SDK_FILES.bin += [
     '!xpidllex.py',
     '!xpidlyacc.py',
     'header.py',
     'typelib.py',
     'xpidl.py',
 ]