This extension is developed by Kitco, a mining services company that is constantly in the business of producing gold. It is therefore safe to assume that they are not trying to sell you a piece of a market but rather providing you with useful tools that you can make use of while getting an important financial data.
The conversion into the form of a Firefox add-on is a compromise as it is not an app. Some very useful features such as full access to pages or plugins are missing. Still 50e0806aeb