I hope Mandarin Pop-up gets updated to FF 4 when it comes out, because this extension is the only reason I ever go back to FF 3.6 these days. I've tried almost all of the Chinese dictionaries for FF, and several stand alone programs, and this one is the definitely the best.
Having the main pop-up feature for unfamiliar characters is great enough, but where this extension in particular shines is the searchable dictionary you can use during any occasion. Even when you don't know exactly what word you are searching for, half-guessing the pin yin is enough to yield a list of results to look through.
Best of all, the dictionary includes measure words in parentheses next to the dictionary definitions, which is a huge help, and something a lot of other dictionaries are missing.