This information is from https://support.mozilla.org/en-US/kb/search-web-address-bar .
Turning off the web search in the address bar
You can turn off the web search in the address bar by changing a preference.
Note: This preference does not affect smart keyword searches from the Location Bar.
- In the Location bar, type about:config and press EnterReturn. (The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise! to continue to the about:config page. )
- In the FilterSearch field, type keyword.enabled.
- Double-click the keyword.enabled preference to set its value to false.