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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
ReminderFox displays and manages lists of reminders and ToDo's. ReminderFox makes sure you remember all of your important dates via easy-to-use lists, alerts, and alarms, right in your browser without the need for a separate calendar program.
FoxClocks shows times around the world - or just your local time - at the bottom of your browser. It deals with daylight saving time so you don't have to.
Encrypt the web! HTTPS Everywhere is a Firefox extension to protect your communications by enabling HTTPS encryption automatically on sites that are known to support it, even when you type URLs or follow links that omit the https: prefix.
Currency exchange rates in the status bar (data from Yahoo!Finance or Google on option).
Display currency graphs.
Get international weather forecasts from AccuWeather.com and display them in any toolbar or statusbar with this highly customizable and unobtrusive extension.