Review for WikiLook by Scott Welker
Rated 5 out of 5 stars
Outstanding tool. Clean and simple interface.
Much better than the others I evaluated: "Merriam-Webster 20080326" and "Dictionary Lookup Extension 1.5.1"
FYI: I was able to achieve a similar capability in IE through IE7Pro's, Preferences, Search. VERY handy!
To create your own collections, you must have a Mozilla Add-ons account.