"I recently discovered a very handy Firestorm feature that changed the way I interact with my inventory. If you open your Inventory and click on the gear icon Firestorm will show additional options for your inventory. The options Add objects on double click and Add clothes on double click are unchecked by default. With these options unchecked Firestorm replaces any items already attached to the position targeted by the item that you double click. With these options checked, however, Firestorm adds the item that you double click, on top of any other items already attached to the targeted position." Read more on the blog.