Personas are free, easy-to-install "skins" for Firefox. Personas Plus extends that built-in functionality to give you even more control and easier access to new, popular, and your own favorite Personas.
United States English Spellchecker for Firefox, Thunderbird and SeaMonkey by Kevin Atkinson and Geoff Kuenning.
To enable, right-click in a multi-line text box, select "Languages", then "English (United States)".
StumbleUpon is the easiest way to discover new and interesting photos, videos, web pages and more, recommended by a community of experts. Tell us what you are interested in and we recommend page after page of amazing stuff you’d never expect to see.
Shareaholic is the easiest way to share interesting links using Facebook, Twitter, Google Mail, Reader, Bookmarks, Evernote, Bitly, StumbleUpon, and more... from one simple ALL-IN-ONE add-on. This is the ultimate tool for the link sharing addict!
Answers.com’s 1-Click Answers ™ saves you time by instantly delivering the information you’re looking for. Point at any word, hold the Alt key (Ctrl in Linux) and click. A pop-up will appear on the screen, explaining the term.
Note from Mozilla: This add-on has been discontinued. Try Evernote instead.
Clipmarks lets you clip and share specific parts of articles, or anything else you read.
Collections are groups of related add-ons that anyone can create and share.Explore Collections