I use 2 themes on Firefox. The only site they are available is addons.mozilla.org. They are using code which does not allow browsers to right click on the install link and select "Save link as..." I have even tried this using Internet Explorer as well. I would like to download each theme as a file and save it to hard disk as a "backup" in case I ever want to install it again sometime, on a different profile.
Here are links to the 2 themes (no affiliation!!):
https://addons.mozilla.org/en-US/firefo ... ack-shine/
https://addons.mozilla.org/en-US/firefo ... bon-fibre/