For some reason my Firefox default search engine has been set to “Microsoft’s Bing” search engine, I wanted to set it back to the default “Google”, so this is what I did after some research
I accessed the config url “about:config” from Firefox and modified the following values

browser.search.defaultenginename : Google
browser.search.defaulturl : http://www.google.com/search?hl=en&q=
keyword.URL : http://www.google.com/search?hl=en&q=