That's not a Sabayon issue. It's an issue with the website's domain status. Manually download the addon package to your system and install it in FF that way. To manually download, just RIGHT click on this link -->
https://addons.mozilla.org/downloads...n-installation <-- and choose "Save Link As". Save it in your downloads directory on your Sabayon system or wherever you normally download stuff. Go there after the file is downloaded, RIGHT click on it and Open With... Firefox.
That should do it for you.
Luck!