It's still not quite at the level or speed of Google Dictionary (by Google) in the Chrome Browser, but it's OK. I think it has to do with integration; that is, if the Firefox team created a built-into the browser version, rather than it being an add-on or extension, it would be more fluid. Since Chrome Browser has built-in spell check, I'm guessing it has some connection to that interface type. Why else would their pop-up results be so instant? No really, why?