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When you select a word on a webpage, right click it and select FreedDictionaryLookup it will pop up a panel with the definition for you. Ctrl-1 and CMD-1 can be used as well after selecting the word. Double clicking a word can select it quickly. CTRL-SHIFT + 1 will open your panel in a new tab.
TheFreeDictionary.com is perhaps one of the best dictionary tools. Not only can you look up a definition, you can easily access images of your word as well, if applicable.
This panel simply allows faster access when doing lots of research on topics on the web without having to constantly open new tabs.
You can also left click and drag any link inside of the panel to your Firefox address bar and drop it in to navigate directly there.
Hope you like this extension. Please review it if you have time.