Spoofstick is available for Firefox and Internet Explorer. If you want this add-on, it is necessary to have a compatible browser. In Firefox, you can install it and enable it from the Add-ons menu. If the toolbar does not appear, you can right click the title bar of the Firefox and select it from that location.
In Internet Explorer, the add-on is enabled in the same way, only that you also need to check Manage Add-ons from the Tools menu.