--- a/python/mozbuild/mozbuild/backend/tup.py
+++ b/python/mozbuild/mozbuild/backend/tup.py
@@ -185,17 +185,17 @@ class TupOnly(CommonBackend, PartialBack
         )
 
         # These are 'group' dependencies - All rules that list these as an output
         # will be built before any rules that list this as an input.
         self._installed_idls = '$(MOZ_OBJ_ROOT)/<installed-idls>'
         self._installed_files = '$(MOZ_OBJ_ROOT)/<installed-files>'
 
     def _get_backend_file(self, relobjdir):
-        objdir = mozpath.join(self.environment.topobjdir, relobjdir)
+        objdir = mozpath.normpath(mozpath.join(self.environment.topobjdir, relobjdir))
         if objdir not in self._backend_files:
             self._backend_files[objdir] = \
                     BackendTupfile(objdir, self.environment,
                                    self.environment.topsrcdir, self.environment.topobjdir)
         return self._backend_files[objdir]
 
     def _get_backend_file_for(self, obj):
         return self._get_backend_file(obj.relobjdir)
@@ -380,19 +380,19 @@ class TupOnly(CommonBackend, PartialBack
                 raise Exception("Cannot install to " + target)
 
         if target.startswith('_tests'):
             # TODO: TEST_HARNESS_FILES present a few challenges for the tup
             # backend (bug 1372381).
             return
 
         for path, files in obj.files.walk():
-            backend_file = self._get_backend_file(mozpath.join(target, path))
             for f in files:
                 if not isinstance(f, ObjDirPath):
+                    backend_file = self._get_backend_file(mozpath.join(target, path))
                     if '*' in f:
                         if f.startswith('/') or isinstance(f, AbsolutePath):
                             basepath, wild = os.path.split(f.full_path)
                             if '*' in basepath:
                                 raise Exception("Wildcards are only supported in the filename part of "
                                                 "srcdir-relative or absolute paths.")
 
                             # TODO: This is only needed for Windows, so we can