--- a/python/mozbuild/mozbuild/mozinfo.py
+++ b/python/mozbuild/mozbuild/mozinfo.py
@@ -150,12 +150,11 @@ def write_mozinfo(file, config, env=os.e
     """Write JSON data about the configuration specified in config and an
     environment variable dict to |file|, which may be a filename or file-like
     object.
     See build_dict for information about what  environment variables are used,
     and what keys are produced.
     """
     build_conf = build_dict(config, env)
     if isinstance(file, basestring):
-        with open(file, "w") as f:
-            json.dump(build_conf, f)
-    else:
-        json.dump(build_conf, file)
+        file = open(file, 'wb')
+
+    json.dump(build_conf, file, sort_keys=True, indent=4)