Friendbar makes it easy to keep in touch with friends on Facebook and Twitter. It displays a running stream of text and photo updates from Facebook and Twitter right on the toolbar, and allows you to post updates and reply from the toolbar itself.
Note
Firefox 3.0 and 3.0.* Windows 2000/2003/XP/Vista

CloudBerry Twitter plug-in for Firefox helps you to post excerpts from the websites you visit with a click of the button and attach a short URL to the source page.

* Select the text on the web pages and tweet it to your friends
* Add link to the source with Chilp.It
* Install in a few minutes
* Download for free
* No registration required

Note
Firefox 1.5 – 3.0.*

CloudBerry Twitter plug-in for IE helps you to post excerpts from the websites you visit with a click of the button and attach a short URL to the source page.

* Select the text on the web pages and tweet it to your friends
* Add link to the source page with TinyURL
* Add link to the source with Chilp.It
* Install in a few minutes
* Download for free
* No registration required

Note
.NET Framework 2.0 or higher. IE 6 or IE 7.

Power Twitter was designed to make the Twitter experience more full-bodied. The folks at Twitter are building a core competency in storing and delivering messages and understandably have been reluctant to get too bogged down in product design. This has allowed 3rd parties to build a great set of Twitter clients. Power Twitter is a little bit different because it interacts directly with the Twitter.com website. And because it is embedded in the browser, Firefox, you are able to do some pretty slick things!
Note
Firefox 1.5 – 3.0.*

TwitterFox is a Firefox extension that notifies you of your friends’ tweets on Twitter. This extension adds a tiny icon on the status bar which notifies you when your friends update their tweets. Also it has a small text input field to update your tweets.
Note
Firefox 2.0 – 3.1b2