Echofon (formerly TwitterFox) is much smaller than Tweetdeck and it’s one of mozilla firefox’s extension.
Keep up with Twitter while you browse the web using Firefox.
TwitterFox is a Firefox extension that notifies you of your friends’ tweets on Twitter.
It adds a tiny icon to the status bar. It notifies you when friends post tweets and has a text entry field for posting your own tweets.
Download Echofon