The writing is a bit ambiguous here, but "true" is the default setting in Firefox nightlies currently, so no about:config tweaks needed. The preference is also

The writing is a bit ambiguous here, but "true" is the default setting in Firefox nightlies currently, so no about:config tweaks needed. The preference is also available in the preferences UI in the "Privacy" section. Of course, most people are probably not running Firefox nightlies either.