Twitter has announced few months ago to developers who used Twitter API to integrate their applications to Twitter that they will shut down the Basic Auth method for authenticating user's credentials and moving on to OAuth.
Today, they officially announced that they have completely shut down all Basic Auth support, meaning that if your Twitter account still uses Basic Auth, it will not work anymore. You must upgrade your clients in order to be able to connect to Twitter again.
Some clients that i have known has updated their code in order to work with OAuth. Unfortunately, Echofon for Firefox has dropped Linux support. It's a shame. I really like Echofon. Right now, i'm using QTwitter and Choqok for my Twitter/Identi.ca accounts. Both works great, so i'm quite happy with it.