No source code is available. Terms of service on the website imply that this is not free software at all. Might send private data online. Should not be available for Firefox as it breaks the terms of service. Reported.
Reply: that is not how you provide the source. In Firefox, there is no direct way to download the latest version from that URL: only install it. Your terms of service explicitly disallow "unzipping" the archive anyways: "under no circumstances User will use the Service and Service Data to [...] reverse engineer, decompile, disassemble, or otherwise attempt to discover any source code, object code or underlying structure, ideas or algorithms of the Service or any software, documentation or data related to or provided with the Service."
And note that I did not say "analytics", I said "might send private data online" and other reviews here have reported the plugin does not work offline.