After Firefox updated to version 57, I couldn't use the Bing-Google extension which worked pretty well. https://addons.mozilla.org/en-GB/firefox/addon/bing-google-1/?src=api
This addon does the same job, so it is looks like a good replacement. I use it with Cortana which otherwise redirects me to Bing, but I do not like Bing. Good by, Bing! Thank you, developer wools.
P.S. Also, it does not seem to require any other software, like the previous extension which required to install another program.