The login to Stack Exchange sites on Firefox only possible if you allow cookies from "not visited 3rd party sites".
That's not so nice I think, because normally you won't ever accept cookies from sites you don't visit yourself. Is there a way to do the login without allowing this?
To reproduce: In Firefox, go to
preferences -> privacy -> user defined history -> accept third-party cookies: From visited.
Then I can't login to Stack Overflow.