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For Mozilla Firefox, the Google Translator plugin by Andy Portmen works well to translate highlighted text using the Google Translate service or entire pages via the icon it installs on the menu bar. This tool can also be used by using the contextual menus via the mouse.

If you're looking for a browser plugin that uses Microsoft Translate service, Microsoft has a cross-browser plugin called "Translator bookmarklet" at http://labs.microsofttranslator.com/bookmarklet/ which translates a webpage in place without leaving the page. Works in Mozilla Firefox, Google Chrome and Internet Explorer.

There is also a really great plugin for Mozilla Thunderbird called Quick Translator that works really well at translating highlighted text using Google Translate. It can be found here, at https://addons.mozilla.org/en-US/thunderbird/addon/quick-translator/ or if using Thunderbird by searching for it either under extensions or add-ons. 

Microsoft Outlook

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