MAGETWO-47643: Data Grid float action bar continuously appears & disappears in Mozilla Firefox

- Fixed action bar issue

Author: Oleksandr Dubovyk

app/design/adminhtml/Magento/backend/Magento_Backend/web/css/source/module/main/_actions-bar.less

@@ -30,6 +30,10 @@
         .store-switcher {
             display: none;
         }
+        
+        .page-actions-placeholder {
+            min-height: 50px;
+        }
     }
 }