--- a/python/mozbuild/mozbuild/backend/tup.py
+++ b/python/mozbuild/mozbuild/backend/tup.py
@@ -158,16 +158,32 @@ class BackendTupfile(object):
 
 class TupOnly(CommonBackend, PartialBackend):
     """Backend that generates Tupfiles for the tup build system.
     """
 
     def _init(self):
         CommonBackend._init(self)
 
+        self._supported_dirs = (
+            'services',
+            'servo',
+            'startupcache',
+            'storage',
+            'taskcluster',
+            'testing',
+            'third_party',
+            'toolkit',
+            'tools',
+            'uriloader',
+            'view',
+            'widget',
+            'xpcom',
+            'xpfe',
+        )
         self._backend_files = {}
         self._cmd = MozbuildObject.from_environment()
         self._manifest_entries = OrderedDefaultDict(set)
         self._compile_env_gen_files = (
             '*.c',
             '*.cpp',
             '*.h',
             '*.inc',
@@ -252,17 +268,17 @@ class TupOnly(CommonBackend, PartialBack
             self._process_final_target_pp_files(obj, backend_file)
         elif isinstance(obj, JARManifest):
             self._consume_jar_manifest(obj)
         elif isinstance(obj, PerSourceFlag):
             backend_file.per_source_flags[obj.file_name].extend(obj.flags)
         elif isinstance(obj, ComputedFlags):
             self._process_computed_flags(obj, backend_file)
         elif isinstance(obj, (Sources, GeneratedSources)):
-            if obj.relobjdir.startswith('xpcom'):
+            if obj.relobjdir.startswith(self._supported_dirs):
                 backend_file.sources[obj.canonical_suffix].extend(obj.files)
 
         return True
 
     def consume_finished(self):
         CommonBackend.consume_finished(self)
 
         # The approach here is similar to fastermake.py, but we
@@ -428,17 +444,17 @@ class TupOnly(CommonBackend, PartialBack
                                  destdir=mozpath.join(self.environment.topobjdir, obj.install_target, path))
 
     def _process_computed_flags(self, obj, backend_file):
         for var, flags in obj.get_flags():
             backend_file.local_flags[var] = flags
 
     def _process_unified_sources(self, obj):
         backend_file = self._get_backend_file_for(obj)
-        if obj.relobjdir.startswith('xpcom'):
+        if obj.relobjdir.startswith(self._supported_dirs):
             files = [f[0] for f in obj.unified_source_mapping]
             backend_file.sources[obj.canonical_suffix].extend(files)
 
     def _handle_idl_manager(self, manager):
         if self.environment.is_artifact_build:
             return
 
         dist_idl_backend_file = self._get_backend_file('dist/idl')