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Instantbird is a customizable instant messaging client with support for AOL, Yahoo, MSN, Google Talk, Twitter, Facebook, XMPP, IRC and ICQ. It's powered by Mozilla so it has support for customization using themes and extensions built-in.

Instantbird is designed to make instant messaging better:

Talk to all your friends - With Instantbird, you can now talk to all your friends with a single application. AIM, Yahoo, MSN, Google Talk, Facebook and even Twitter are supported.
Trouble free - Instantbird has been designed with stability and simplicity in mind. You will enjoy its clean clutter-free user interface: nothing will distract you from your conversations. No ads, you won't be annoyed by unwanted content.
Conversations in Tabs - Conversations are grouped in Tabs, ensuring you never feel swamped by conversations windows poping up everywhere. The tabs are easy to use: you can drag them to reorder them, or move a conversation into a new window.
Customizable conversations - With a few carefully designed message themes available by default and hundreds more available as add-ons, there's all you need to make your conversations feel personal. Let's turn them into a colorful, delightful experience!
Smart contacts - Having several ways to contact the same person? No problem, just drop a contact onto another and Instantbird will know it's the same person.
Always here for you - In addition to the portable Windows version, standard Instantbird can work on all your computers, whether they are running Windows, Mac OS X or Linux. Being free and Open-Source, you will never have to pay to use it. You will never have to learn to use another IM client again.
Limitless extensibility - Powerful tools like inline search or zoom are built-in to help you get the most out of your conversations. More are available as add-ons, thanks to Instantbird's limitless extensibility.