I have firefox and I would like thesaurus and synonyms

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I use to have that until I upgraded but I don't know if it was the last upgrade it became missing or the upgrade before that upgrade but I did have it. I want to be able to click on a word and get the synonyms for that word. I checked firefox add ons in exstentions but couldn't find it. I would even change my browser if you know of a browser that supports that service. Please Help!