OneLook (all dictionaries) - Firefox Addon OneLook (all dictionaries) - Firefox Addon - The OneLook website indexes word definitions from a whole host of online dictionaries, meaning that if one dictionary doesn't have the definition you are looking for, OneLook's search might well find the word definition in another dictionary, too
Google Translate It - Firefox Addon Google Translate It - Firefox Addon - This jetpack allows you to quickly translate text with Google Translate.
Currently only a widget is added that when clicked will change the page to a Google Translated version of the page
Dictionary.com - Firefox Addon Dictionary.com - Firefox Addon - This addon searches for words using Dictionary.com, a free online service. Dictionary.com will give definitions from many different sources, including Merriam-Webster and American Heritage Dictionary, along with Dictionary.com Unabridged
DiiCT (Multilingual Dictionary) - Firefox Addon DiiCT (Multilingual Dictionary) - Firefox Addon - This addon searches for translations using DiiCT.com, a free online multilingual dictionary. DiiCT.com currently supports translation between the following languages: English, German, French, Italian, Spanish, Dutch, Portuguese, Czech, Polish, Russian and Armenian.
The dictionary DiiCT