LinkedIn is a social network whose purpose is to connect professionals and classmates. It allows you to expand your career opportunities, find and network with colleagues, and ask your peers any questions pertaining to your industry. LinkedIn Companion is an addon that allows you to connect LinkedIn to Firefox.
Yoono’s tagline is “Socialize your Browser.” And it does just that. Yoono allows you to connect Firefox to popular social networks such as AIM, Facebook, Twitter, MSN, and others and chat with your friends wherever they are — or discover and share content with them.
I used to use ChatZilla back in 2004 when it was a little bit more buggy, and a bit less popular, when it was part of the original Mozilla Suite, which has been replaced by the oddly named SeaMonkey. However, the project has definitely grown since then.