Dictionary Anywhere for Firefox is a handy little add-on that provides word definitions directly from wherever you are reading.
Instead of leaving the web page and bringing up a Google search, why not just view the definitions as you browse the web. To bring up a definition, you can double-click any word, and the results are displayed in a small pop-up bubble. It supports Spanish, German, French, and English. There is also (when available) an option to hear the word spoken right from the bubble. Additionally, if you require more details, you can click on the word 'More,' and it will bring up a Google search with your target word already filled in so you can find synonyms, antonyms, etc.
Dictionary Anywhere will be a useful asset while reading new subject matter that requires your attention because you stay on the current page you are reading rather than navigating elsewhere.