A while ago I put up this article, reviewing 5 so-called 'essential' add-ons for Firefox. To continue in the same manner in this second part, here are 5 add-ons updated for Firefox 3.5 which can prove useful. Maybe not the most popular, but they definitely deserve a try.
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