Definitions.net per STANDS4

Definitions.net is a free multilingual dictionary that provides instant definitions from many respected reference resources such as the Random House College Dictionary, Princeton WordNet, Wiktionary, Webster Dictionary, U.S. National Library of Medicine, DOD Dictionary of Military and Associated Terms and more.

To use this extension simply select a word on any web page you're visiting and click on the extension icon. A pop-up window will open with all possible definitions for the selected word.

Note: The extension will not work in tabs that were open prior to installation. After installing this extension, either reload your open tabs or restart Firefox.