Read the Update below.

An excellent enhancement extension for Firefox Quantum. I use to use DictionarySearch to do the same thing but that addon had very limited functionality compared to the much more powerful SSS. However, using this with 59.0.2 doesn't allow you to Open a Search in a New Tab or Window for some reason though the options are available within SSS. A bug perhaps with the latest SSS and/or Firefox? Otherwise i would be rating this 5 stars instead of 4.

Update: changed my original rating from 4 stars to 5 stars because the developer worked with me closely in troubleshooting the issue I had and helped me resolve it. It was my own folly... I needed to change the Open New Tab behavior in the Popup behavior menu and not the Context Menu options. The extension now works perfectly. My first 5 star rating for the new age of Quantum. All hail SSS!