If Firefox is working correctly and you click on the theme, a page should come up where it says in a green box Add Theme. Click on that and then OK and the theme should be up in your browser. Restart Firefox and see if that works if not, something is wrong with your settings. Because the themes do work.
Pretty, soothing, and a reminder of what the beautiful nature of the US of A looks like! This theme will give you a positive focus, despite the lack of love, ethics, and mutuality morality, of politics or other planet Earth concerns. <3