--- a/toolkit/mozapps/installer/l10n-repack.py
+++ b/toolkit/mozapps/installer/l10n-repack.py
@@ -104,43 +104,42 @@ def repack(source, l10n):
 
     # For chrome and non chrome files or directories, store what langpack path
     # corresponds to a package path.
     paths = dict((e.path,
                   l10n_paths[mozpack.path.basedir(e.path, bases)][e.name])
                  for e in entries if isinstance(e, ManifestEntryWithRelPath))
 
     for path in NON_CHROME:
-        for p, f in l10n_finder.find(path):
+        left = set(p for p, f in finder.find(path))
+        right = set(p for p, f in l10n_finder.find(path))
+        for p in right:
             paths[p] = p
+        for p in left - right:
+            paths[p] = None
 
     # Create a new package, with non localized bits coming from the original
     # package, and localized bits coming from the langpack.
     packager = SimplePackager(formatter)
     for p, f in finder:
         if is_manifest(p):
             # Remove localized manifest entries.
             for e in [e for e in f if e.localized]:
                 f.remove(e)
-        base = mozpack.path.basedir(p, paths.keys())
-        if base:
-            # If the path is one that needs a locale replacement, use the
-            # corresponding file from the langpack.
-            subpath = mozpack.path.relpath(p, base)
-            path = mozpack.path.normpath(mozpack.path.join(paths[base],
-                                                           subpath))
-            files = [f for p, f in l10n_finder.find(path)]
-            if len(files) == 0 and base in NON_CHROME:
-                path = path.replace(locale, l10n_locale)
+        if p in paths:
+            path = paths[p]
+            if path:
+                # If the path is one that needs a locale replacement, use the
+                # corresponding file from the langpack.
                 files = [f for p, f in l10n_finder.find(path)]
-            if len(files) == 0:
-                if not base in NON_CHROME:
-                    errors.error("Missing file: %s" % os.path.join(l10n, path))
-            else:
-                packager.add(path, files[0])
+                if not len(files):
+                    if base not in NON_CHROME:
+                        errors.error("Missing file: %s" % os.path.join(l10n, path))
+                else:
+                    packager.add(path, files[0])
         else:
             packager.add(p, f)
 
     # Add localized manifest entries from the langpack.
     l10n_manifests = []
     for base in set(e.base for e in l10n_entries):
         m = ManifestFile(base)
         for e in l10n_entries: