--- a/layout/tools/reftest/reftestcommandline.py
+++ b/layout/tools/reftest/reftestcommandline.py
@@ -129,16 +129,24 @@ class ReftestArgumentsParser(argparse.Ar
                           "The extension file's name should be <id>.xpi where <id> is "
                           "the extension's id as indicated in its install.rdf. "
                           "An optional path can be specified too.")
 
         self.add_argument("--marionette",
                           default=None,
                           help="host:port to use when connecting to Marionette")
 
+        self.add_argument("--marionette-port-timeout",
+                          default=None,
+                          help=argparse.SUPPRESS)
+
+        self.add_argument("--marionette-socket-timeout",
+                          default=None,
+                          help=argparse.SUPPRESS)
+
         self.add_argument("--setenv",
                           action="append",
                           type=str,
                           default=[],
                           dest="environment",
                           metavar="NAME=VALUE",
                           help="sets the given variable in the application's "
                           "environment")
@@ -336,16 +344,22 @@ class DesktopArgumentsParser(ReftestArgu
             if options.totalChunks is not None or options.thisChunk is not None:
                 self.error(
                     "cannot specify thisChunk or totalChunks with parallel tests")
             if options.focusFilterMode != "all":
                 self.error("cannot specify focusFilterMode with parallel tests")
             if options.debugger is not None:
                 self.error("cannot specify a debugger with parallel tests")
 
+        if options.debugger:
+            # valgrind and some debuggers may cause Gecko to start slowly. Make sure
+            # marionette waits long enough to connect.
+            options.marionette_port_timeout = 900
+            options.marionette_socket_timeout = 540
+
         if not options.tests:
             self.error("No test files specified.")
 
         if options.app is None:
             bin_dir = (self.build_obj.get_binary_path() if
                        self.build_obj and self.build_obj.substs[
                            'MOZ_BUILD_APP'] != 'mobile/android'
                        else None)

--- a/layout/tools/reftest/runreftest.py
+++ b/layout/tools/reftest/runreftest.py
@@ -597,24 +597,27 @@ class RefTest(object):
                             env=env,
                             process_args=kp_kwargs)
         runner.start(debug_args=debug_args,
                      interactive=interactive,
                      outputTimeout=timeout)
         proc = runner.process_handler
 
         if self.use_marionette:
-            marionette_args = { 'symbols_path': options.symbolsPath }
+            marionette_args = {
+                'socket_timeout': options.marionette_socket_timeout,
+                'symbols_path': options.symbolsPath,
+            }
             if options.marionette:
                 host, port = options.marionette.split(':')
                 marionette_args['host'] = host
                 marionette_args['port'] = int(port)
 
             marionette = Marionette(**marionette_args)
-            marionette.start_session()
+            marionette.start_session(timeout=options.marionette_port_timeout)
 
             addons = Addons(marionette)
             if options.specialPowersExtensionPath:
                 addons.install(options.specialPowersExtensionPath, temp=True)
 
             addons.install(options.reftestExtensionPath, temp=True)
 
             marionette.delete_session()