This extension is available for download on Mozilla Addons page.

Popup Encyclopedia is a blazingly fast dictionary/encyclopedia which can be used to search any word or collection of words. It supports double-click-on-text navigation. Double-click-on-text navigation is a feature by which you can double-click on any text in the popup window to get it's meaning without having to type it again in search box. [ Note: As of Ver 2.2, this feature is only available on the toolbar popup. ]

The features are:

• Works offline as a dictionary.
• Supports word-lemmatization and word-stemming to increase accuracy.
• Clean and minimalistic UI.
• Double-click-on-text navigation.
• Cross platform extension. Works well in Firefox, Chrome and Opera.
• For keyboard ninjas, ALT + SHIFT + M is the shorcut key.

For demo see [here]
For screenshots see [here]

./stemmer/PorterStemmer1980.js - 3-clause-BSD License
./popup/dictionary.txt - GNU GPL2
popup icon - Apache License ( part of Google's Material Icons ).
./popup/lookup.txt - mentioned in ./popup/LOOKUP_LICENSE.txt
All the files excluding above are under MIT License

• https://www.mediawiki.org/wiki/API:Main_page [link]
  
• https://github.com/matthewreagan/WebstersEnglishDictionary/blob/master/dictionary.json [link]
  
• https://github.com/explosion/spaCy [link]
  
• https://github.com/kristopolous/Porter-Stemmer [link]