Described as the Facebook of the business world, with others in your profession. If you're a fan of the site then the official LinkedIn Companion for Firefox extension is a must.

The add-on does three main things. The first is to add a button beneath the bookmarks toolbar that displays a menu, showing shortcuts to various sections of the LinkedIn Web site. It also allows you to share the Web page you're browsing on your LinkedIn profile, and tracks recently-viewed profiles.

Secondly, the extension adds LinkedIn entries to the Firefox search box so you can quickly search both the LinkedIn site and also its jobs section.

Thirdly, and arguably most useful of all, the add-on places a small LinkedIn icon alongside the name of contacts within Web mail services, such as Gmail and Hotmail. Clicking this then shows a pop-up window with a short précis of their LinkedIn profile, if they have one. Clicking the link will display their whole profile and you can add them as a contact.