An extension for Firefox lets you enter the Facebook and Twitter accounts

[maripuri]

Firesheep, an extension for Mozilla Firefox browser, allows users to access accounts from different sites such as Facebook, Twitter, Windows Live, Flickr, Google, WordPress, Yahoo or Amazon. When a user enters one of these pages, enter your user name and password. Once done, the server checks for an account and match these data, if any, replies with a cookie that the browser used on subsequent visits.

You can read more about this extension on the link below

firesheep