Sure, it works, but there are two problems: (1) it only works on-line, that is, it looks up for synonyms on some (unspecified) server (or servers), as it doesn't come with an off-line, built-in dictionary, and (2) it doesn't say so in the extension's description, that is, that it does not inform how it works, that it uses external services to pick the results. For all purposes, it's much less useful than simply opening another tab to look words up on some on-line dictionary.

https://github.com/hrai/thesaurus-extension/issues/3