The addon was uploaded to AMO and signed there. In case you want to install it on another browser or system, there is a Save to Disk option where you can save the addon either in it’s original, unsigned, or signed version on your local hard drive. This is only good for testing new addons. When you install the addon temporarily, it will be automagically removed from the browser once you close Firefox. Why? Because you want to test the addon and see how it goes and whether it is really useful to you, or whether it is working as intended. The third option is to install the addon temporarily.
