Best/quickest dictionary plugin for firefox

[HeXploiT]

A while back I had a dictionary plugin for firefox that I THOUGHT was from MW. It was very quick and all that was needed was a double click on the word and a small box would pop up with the definition. I can't seem to find this now however and I don't remember what it was called. All the other dictionaries I'm finding seem to require outlining the word and right clicking to get the definition.
Anyone have a suggestion for a quicker dictionary or have an idea where I can find this one from merriam webster?

 

[Sam25]

I use this dictionary add-on from Merriam-Webster. Infact this was the only add on from MW that I found. There are also other good dictionary add-ons here.