I use this all the time. I've found it more reliable that using the three finger gesture on the mac, which sometimes selects the incorrect word or doesn't work. Also easier to used that control-commmand-d. Further, it calls the full dictionary, and not the popup. I hope it's updated for Quantum!
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!
This review is for a previous version of the add-on (0.2).