Some Addons are licensed under GPL. This license requires the source to be made available with the binary. When browsing a GPL AddOn such as AffiliateFox, there is no source available. Where can I find the source if the author’s homepage is gone?
It’s best to contact the author and ask them.
In the concrete case, the provided domain in the plugin page is not owned by the author any more and the email address is also invalid.
In the general case, GPL §3 is very strict. It says: “You may […] distribute the Program […] provided that you also do one of the following: […] Accompany it with the complete corresponding machine-readable source code […].”
However, I think, one can extract the source out of the .xpi file somehow…
Just open them with an archiver. I think you can even rename it to .zip and open it through explorer ( on Windows at least ).