diff --git a/testing/mozbase/mozhttpd/mozhttpd/mozhttpd.py b/testing/mozbase/mozhttpd/mozhttpd/mozhttpd.py
--- a/testing/mozbase/mozhttpd/mozhttpd/mozhttpd.py
+++ b/testing/mozbase/mozhttpd/mozhttpd/mozhttpd.py
@@ -70,16 +70,17 @@ class RequestHandler(SimpleHTTPRequestHa
     proxy_host_dirs = False
     request_log = []
     log_requests = False
     request = None
 
     def __init__(self, *args, **kwargs):
         SimpleHTTPRequestHandler.__init__(self, *args, **kwargs)
         self.extensions_map[".svg"] = "image/svg+xml"
+        self.extensions_map[".mjs"] = "application/javascript"
 
     def _try_handler(self, method):
         if self.log_requests:
             self.request_log.append(
                 {"method": method, "path": self.request.path, "time": time.time()}
             )
 
         handlers = [
