Social
AddThis -- Bookmark and share web pages on social sites
Cooliris -- Makes browser full-screen 3D Wall for searching, viewing and sharing the Web
Shareaholic -- Share interesting links on almost all social sites
Yoono -- Don't open each site individually. Stay connected to your social networks
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Screenshots
LightShot -- Fastest screenshot tool of entire page or selected area
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Bookmarking
FastDial-- Bookmark Visually. Right-click any site page and choose "Add to Fast Dial" from the context menu.
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Fast Browsing
CoolPreviews -- Browse, Preview and share links Faster without leaving your current page
WikiLook -- Wikipedia and Wiktionary look up. Fast and light tool that define words for you without need to open new tabs or extra windows.
FastestFox -- Search
selected text from Wikipedia in a second
Fastest Search -- Find as you type text search on page/in all tabs. Instant web/history search in URL bar. Multi-tab preview for selected text.
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Backup
FEBE -- Backup your Firefox data and all favorite addons
Xmarks Sync -- Save your all firefox data in an account and access from any where
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FoxTab -- Its a 3D tab management and best Tab Switching in Firefox.
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Security
AdBlock Plus -- Stop annoying adverts, tracking and bothered banners
WOT -- Web Of Trust shows you which websites you can trust based on millions' of users experiences, so you can enjoy your surfing 100%
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Development
Page Hacker -- Edit web pages, use as a text editor and save the edited page
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Downloading
FlashandVideoDownload -- Download flash games and videos with a single click.
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Miscellaneous
Print Pages to PDF -- Creates Pdf from any number of open Browsertabs, Bookmarks and pages. This document can be archived, sended by e-mail or printed out with any standard Pdf Viewer.
QuickTranslator -- Quick translation in many languages with a click of a mouse.
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Thunderbird Addons
CheckandSend -- Check the message contents and email addresses before you send the message
Clippings -- Save frequently-entered text and paste later when you required
GathertheSenders -- Select all the mails in a directory sent by the selected author.
Forward -- Add Inline / As Attachment option to Forward button and Message context menu.
Slideshow -- Displays a slideshow of picture attachments. When email has attached pictures, it displays "View as Slideshow" button next to the attachment. Button click pops a slideshow window with all the images, allowing you to zoom in/out, save.