Mike Demers has come up with a Twitter add-on called Tweetbar for the Firefox and Flock browsers.
I got this info from Asa's blog.
If you haven't heard about Twitter, you can read this excellent post "Twitter - short, sweet and sleek - Let’s get started" by Brajeshwar.
Note: Tones, thanks for pointing out the mistake.
Tags: twitter firefox flock add-on