Add-ins, add-ons, plug-ins – what they mean depend on who’s using the term. When Firefox uses Add-ons, it means “everything third parties can make to extend the functionality of your browser”. This can be extensions that add functionality like ad blocking, themes that change the look, or plug-ins that allow you to play certain file formats like Flash. From what I can see on first sight, Office add-ins are like Firefox’s extensions, that add functionality like removing duplicates in Outlook. But again, the terminology used really depends on the software author.