By default, when you have a RSS icon in any toolbar of your choice, or in the Australis menu, and no feeds are available for subscribing, the icon is just grayed out, becoming useless. This extension hides the icon instead, opening more room for other items. The icon will reappear when Firefox detects available feeds in the current web page.
This addon doesn't change the appearance of your web browser, it just works on your existing RSS toolbar icon. If you don't have one, you can add it easily by right clicking on your toolbar of choice, and selecting Customize.
Important note: in the new Australis interface, the Customize dialog has been moved into a new tab, so if this extension is enabled, your RSS icon will be hidden there. If you need to place the RSS Subscribe button in your toolbar or menu, or if you need to move it around, first you need to disable this extension temporarily!