Good So Far (v 0.2) Rated 3 out of 5 stars
This is a good start. As of version 0.2, it's already simpler than most other dictionary add-ons for Firefox. It's also very lightweight and fast. A couple of suggestions that may make it even better:
1. Currently, the "Look Up in Dictionary" call out is above the "Search Google for..." call out. Most people may search for a phrase or popular news before they look it up in the dictionary, so it seems that "Look Up in Dictionary" should be listed second.
2. In Safari, to look up a word, you can also simply hover over the word and press Control + Command + D on your keyboard, which brings up a call out with the dictionary meaning. That'd be helpful here, too.
Great start! I'm already using this as my default dictionary add-on in Firefox 3.5!