Synonyms.com by STANDS4

Synonyms.com is an online thesaurus reference resource that provides instant synonyms and antonyms definitions in several languages, along with visual diagrams of their associated semantic relationships.

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 synonyms and antonyms 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.