I think there is no social community user who didn't hear about StumbleUpon.com, or at least came across a mention of it on the web. StumbleUpon allows you to share cool links, make friends, drive traffic to your website, rate website and write reviews. It is a huge, growing community for exchanging whatever is cool and hot on the web. This add-on is a powerful extension for Firefox which will bring the entire StumbleUpon experience integrated in your browser. It provides a navigation bar just below the location bar, it allows to log in and like/dislike website. For StumbleUpon users, this is a must-have.
Install StumbleUpon from the Mozilla add-ons page
DownThemAll! (or dTa for short) is a popular download manager for Firefox. dTa also allows to download all the links, video, audio and images embedded in a web page. DownThemAll! comes with plenty of configuration options, like notifications (visual and audio), filter which kind of files should be downloaded, enable/disable context and Tools menu entries, filters. A very powerful extension.
Install DownThemAll! from the Mozilla add-ons page
Selecting and filtering files to download
User Agent Switcher
You know those websites which force you use Internet Explorer in order to access their functionality at full potential, or games etc.? This add-on is a good helper when it comes to those. It allows you to change the way Firefox sends its version name, so you can browse a website with the user agent set to IE6, or IE7 or Opera and so on.
Install User Agent Switcher from the Mozilla add-ons page
Easily switching the Firefox user agent
This add-on will only be useful for Twitter users: it enhances the Twitter experience in Firefox by notifying you of your friends' activity. TwitterFox also provides a small icon in the lower right corner of the Firefox window.
Install TwitterFox from the Mozilla add-ons page
Install Greasemonkey from the Mozilla add-ons page