Super excellent dictionary, thank you. For those having some trouble getting the english-english dictionary to work: here's some more info for mouse dictionary for firefox:
follow all the links, all the information is there.

info for the english-english dictionary is at the bottom of the page.

Download the whole lot, i.e. the zip file
Unzip on your hard drive.
To import the dictionary data, choose the following options:

Character set of dictionary data: UTF-8
Format of dictionary data: JSON
Choose file: dictionary.json

Plus, in the browser extensions navigate to: Manage Extension Shortcuts
to select what keys to activte the plugin, (ALT+Z is suggested)
the shortcut activates it and de-activates it too. Good luck, it's worth the work :)