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ImTranslator provides translation powered by Google Translate, Microsoft, Babylon for 70+ languages. It includes Translator, Dictionary, Text-to-Speech, Back Translation.
Video Tutorial: http://youtu.be/qyLyFkQuMGU
NoSquint allows you to adjust the text-only and full-page (both text and images) zoom levels as well as color settings both globally (for all sites) and per site.
Notifier for gmail, yahoo, hotmail, aol and more webmails.
X-notifier(aka WebMail Notifier) checks your webmail accounts and notifies the number of unread emails...
Supports : gmail, yahoo, hotmail, POP3/IMAP, facebook, twitter and more
The greatest user script engine on the Internet (a fork of Greasemonkey).
Monitors web pages for updates. Useful for websites that don't provide Atom or RSS feeds.
Bring back your personal and powerful Firefox Button!
More Advanced (hidden) configures
Google translate gadget for firefox. This extension translates entire webpages into a language of your choice with one click. Idea comes from Google Translation Bar for Chrome.
Stratiform is the ultimate user friendly customization add-on for Firefox. It puts the power in the average user's hands to customize the browser's look.
If you have multiple logins for web based e-mail accounts (like Gmail and Yahoo! mail), then CookieSwap enables you to easily switch between those different accounts by swapping the 'cookies' that the sites use to know you.
Resize 'Add Bookmark' Popup / 'Edit This Bookmark' Popup, fixes tree selection bug, option to disable popup auto close & Update Url and/or Name of a bookmark with the Url/Title of current page.
Replacement of OpenBook addon
Right double-click anywhere on web page to close current tab.
The only Firefox add-on that allows you to display your Google bookmarks in a hierarchy tree. Display, access and manage Google Bookmarks on Firefox. It also shows the bookmark's favicons.