--- a/testing/marionette/harness/marionette/runner/base.py
+++ b/testing/marionette/harness/marionette/runner/base.py
@@ -432,16 +432,17 @@ class BaseMarionetteArguments(ArgumentPa
             args.app_args.append('-jsdebugger')
             args.socket_timeout = None
 
         args.prefs = self._get_preferences(args.prefs_files, args.prefs_args)
 
         if args.e10s:
             args.prefs.update({
                 'browser.tabs.remote.autostart': True,
+                'browser.tabs.remote.force-enable': True,
                 'extensions.e10sBlocksEnabling': False
             })
 
         for container in self.argument_containers:
             if hasattr(container, 'verify_usage_handler'):
                 container.verify_usage_handler(args)
 
         return args

--- a/testing/marionette/harness/session/runner/base.py
+++ b/testing/marionette/harness/session/runner/base.py
@@ -401,16 +401,17 @@ class BaseSessionArguments(ArgumentParse
             args.app_args.append('-jsdebugger')
             args.socket_timeout = None
 
         args.prefs = self._get_preferences(args.prefs_files, args.prefs_args)
 
         if args.e10s:
             args.prefs.update({
                 'browser.tabs.remote.autostart': True,
+                'browser.tabs.remote.force-enable': True,
                 'extensions.e10sBlocksEnabling': False
             })
 
         for container in self.argument_containers:
             if hasattr(container, 'verify_usage_handler'):
                 container.verify_usage_handler(args)
 
         return args