Delicious users who have recently signed up for a Delicious account with a Yahoo! ID have reported that a "Authentication failed" error message appears when they try to post a URL using the Delicious Post Firefox extension.
As it turns out, the authentication mechanism that is used in the underlying Delicious API is different for users with Yahoo! IDs. The so-called "old skool" Delicious logins still work, however.
A fix for this problem is in the works and will be released soon.