If you type "foo" into firefox's address bar, it'll either assume you meant
http://www.foo.com or will do a Google search for "foo". To turn this off
and have "foo" mean "http://foo" (i.e. a local host named foo):

about:config
browser.fixup.alternate.enabled = false
keyword.enabled = false

from https://support.mozilla.org/en-US/kb/search-web-address-bar