You can rename the extension to .zip and extract. If you have any problems, let me know via a comment or post a contact method and Ill send you the source. Its pretty simple. When on, it intercepts URLs, sends the base URL to three offshore DNS services via HTTP, makes a best effort to check that two of them are equivalent, caches the IP for the browser session, redirects to the equivalent URL using the IP, and substitutes out the domain name in the source code with the IP address for future requests. I admit that it could use some work, however, I wrote in hopes that I could help create some kind of change in the events that are about to transpire before the vote.
xpi files are "normal" renamed zip files. Try extracting the contents using your favorite unzipping application. You should then get access to the source files.