I've been searching for this for months, and the main reason I couldn't find it was because I didn't even use it for a dictionary, but other search engines, which is something it does better and faster than the leading addons in that category. It's the only one I know of that lets you put them in the very first context menu layer, and not in a sub-menu for faster access. Also no extra hassle with downloading search engine toolbars, like many require you to do. It does currently work on latest stable version of Firefox for me with no problems, i can't confirm for betas.
The only suggestions I would give would be picture icons next to each search but it's no big deal, room for more slots of course which i'm sure everyone wants, and change tags or whatever on this site so it pops up more often so other scan discover it(not sure how this works). thanks for one of the most simple and useful addons i've seen