dictextension
A Mozilla extension that looks up words using the DICT protocol
Dict is a Mozilla extension that lets you easily look up the definition of words on a Web page using an online dictionary. Simply select a word, right-click, and you'll see a menu option to define the selected word. The definitions are returned in a new window.
The extension implements a DICT protocol client, defined in RFC 2229. By default, the extension queries the DICT server on dict.org for definitions, but it can be configured to use other servers.
It works on Firefox, SeaMonkey...