Rated 5 out of 5 stars
The below reviewer (Op4) is incorrect—it still works perfectly. I'm using Firefox 50 in the year 2017 and it worked instantly and beautifully. It brings up thesaurus.com when you use it.
The best part is this isn't even an "addon". When you click "Add to Firefox", it just gives you a quick pop-up asking if you want to add it to your list of Firefox search engines. Then it's ready to go!
Can't wait to see what the new synonyms of "Trump" will be over the next year.