Impossible to block access to Twitter.
It is impossible to block access to Twitter through the WebBlocker service or by adding it as a blocked site. This is probably because the registered FDQN is "www.twiter.com" (spelled without the double "t") but the common normal spelling of twitter.com has the double tt.
When you try to access "www.twiter.com" the Firebox WebBlocker will block access.
However, if you try to access "www.twitter.com" (the common spelling of the actual website) it is not blocked by the WebBlocker service and it loads every time.
Also, it is not possible to manually add "www.twitter.com" as a blocked site by FQDN because "twitter.com" is apparently not the actual FQDN.