I often use Google’s “define:” search tag to find definitions of things from various different sources. It is a good way to find several points of view and not just one dictionary definition. That is why I was happy to find the Define extension for Firefox.
Search the definition of the highlighted text at Google. If there is highlighted text, a new option is added to the context menu. This option search the definition of the highlighted text at Google (like searching “define:some_text”).
To use it, all you need to do is highlight a word in your browser, right-click on it and select Define “yourword”. That’s it. Sometimes the more simple extensions out there are my absolute favorite.