How to disable the automatic “www.” prefix in Firefox

To disable Firefox's automatic URL fixup feature, do the following:

Type about:config in address line
Search for browser.fixup.alternate.enabled
Change the value to false
Search for browser.urlbar.autoFill
Change the value to false

https://superuser.com/questions/1246769/disable-automatic-www-insertion-in-firefox

http://www.dusek.eu/how-to-disable-www-prefix-in-firefox/