If domain is WSJ or FT, set referer="facebook.com".
That's it.
EDIT: BTW in case you don't know, Firefox add-on extension .xpi is actually just a zip in disguise. Rename it to .zip and open it as normal. You can then investigate the source. But I agree that it would be neat if the addons.mozilla.org allowed to see this directly on their page. Although on the other hand, this loses usefulness for larger addons.
Because WSJ (and other sites) filter access based on the referer header. They've made a business decision to allow access to people following links from Facebook, likely in the hopes that the readers will find the article useful and be more inclined to subscribe. They also benefit from the views such links generate.
Facebook de-ranks publishers that have paywalls. Since these publishers want Facebook traffic, they don't show paywalls for people that come from Facebook. One way to implement this is by looking at the referer.
If domain is WSJ or FT, set referer="facebook.com".
That's it.
EDIT: BTW in case you don't know, Firefox add-on extension .xpi is actually just a zip in disguise. Rename it to .zip and open it as normal. You can then investigate the source. But I agree that it would be neat if the addons.mozilla.org allowed to see this directly on their page. Although on the other hand, this loses usefulness for larger addons.