--- a/mail/test/mozmill/account/test-retest-config.js
+++ b/mail/test/mozmill/account/test-retest-config.js
@@ -41,16 +41,19 @@ function setupModule(module) {
 // Select File > New > Mail Account to open the Mail Account Setup Wizard
 function open_mail_account_setup_wizard(k) {
   wh.plan_for_modal_dialog("mail:autoconfig", k);
   mc.click(new elib.Elem(mc.menus.menu_File.menu_New.newMailAccountMenuItem));
   return wh.wait_for_modal_dialog("mail:autoconfig", 30000);
 }
 
 function test_re_test_config() {
+  // Opening multiple windows in the same run seems to require letting the stack
+  // unwind before opening the next one, so do that here.
+  mc.sleep(0);
   open_mail_account_setup_wizard(function (awc) {
     // Input user's account information
     awc.e("realname").focus();
     input_value(awc, user.name);
     awc.keypress(null, "VK_TAB", {});
     input_value(awc, user.email);
 
     // Click "continue" button