As the topic states... does such a thing exist? When I googled firefox addons I found some spell-checking addons, but didn't seem to find any real dictionary type of addon where you can just hover your mouse over the word and immediately see an english definition.

[Zgarbas]

Given how rikaisama becomes useless once you go on Japanese character-only websites (or Chinese ones, for the matter), I'd assume that a latin alphabet rikaisama would be way too chaotic to actually implement. Since it can't tell when content is in another language it would try to look up everything in the dictionary. (though it would work, were it aimed at markets where the latin alphabet is not the norm)

https://chrome.google.com/webstore/detai...coja?hl=en

A google pop up dictionary for chrome. This should be what you're looking for.

@OP
Have you looked at those?
https://addons.mozilla.org/fr/firefox/addon/gtranslate/
https://chrome.google.com/webstore/detai...obmjgjcoja

EDIT: ... never mind.

That firefox google translate addon seems to be working pretty well. Thanks!