KitaDic is a minimalism English-English Dictionary add-on for Firefox, and is also one of the least obtrusive dictionary.
Visit KitaDic Addon page on Firefox.
Move your mouse pointer over a word and hold down CTRL(CMD on Mac) key for a second to see its definition. No need to click or select a word before pressing CTRL key.
KitaDic will fetch the meaning of the word from Wiktionary.org , and display it in a Post-it-like window. Further, it provides links to the entries in en.Wikipedia.org, Google dictionary, Answers.com, Naver and Daum.
It provides ultra-easy on/off switch on the bottom-right corner of the Firefox window. Click the smiling face on the status bar to turn KitaDic on/off.