Echofon Twitter client for Firefox redesigned and updated for Firefox 4
Some cool Echofon desktop features have made their way into the Firefox extension this time around, too. It supports muting of users, hashtags, and even specific Twitter clients, as well as saved searches (with a handy new tabbed interface). The very useful conversation view has also been added, allowing you to keep up with @reply chatter more easily.
Echofon for Firefox plans to support real-time tweet streaming through Twitter's user streams API, but that's still pending approval from Twitter. This update is still in Beta, so it might have some issues, but the Firefox extension is now giving standalone Echofon desktop apps a run for their money.