Apparently these (or similar) extensions exist for many languages, as well.
You just have to download the dictionary you want to use (the extension links above have links for those), add the extension to your existing (free) Firefox browser, and you’re set! Particularly helpful when you’re learning a language.
I found this via a Language Log post last night, although that post was just using this as an example of onomatopoeia in Japanese, which is certainly worthy — who doesn’t like onomatopoeia?
A brief quote from that post for your pleasure:
“Although I didn’t know offhand exactly what PERAPERA meant, I could tell from its form that it must be an onomatopoeic term. When I looked PERAPERA up in my little Kenkyusha and Sanseido dictionaries, I discovered that as an adverb it indicates ‘fluently, glibly, volubly,’ as a verb it means ‘prattle, gibber, chatter, gab, rattle (on),’ and as an adjective it describes something that is ‘thin, flimsy, skinny.'”