Firefox 45 has introduced a regression where certain Flash sites, such as online games and live streaming videos, cannot finish loading when the browser is configured to block third-party cookies. Mozilla developers are working on the solution.
Update: It was discovered that the non-Flash Google Inbox Web application was also broken due to the same bug in the code. Corrected this article’s title accordingly. The issue has been solved with Firefox 45.0.1.
Update: Users are reporting the Google Inbox issue has not been fixed with Firefox 45.0.1. Google Sheets and several more sites have also been found broken when third-party cookies are blocked. We have discovered in one of these bugs that
XMLHttpRequest POST requests made in workers do not send the HTTP
Cookie header even if those resources are on the same origin. Mozilla developers are investigating the details.
Update: This bug has been fixed with Firefox 45.0.2.