jquery is not functional?

firefox (v52.0), jquery

this works:

// example 1
$('html').find('body')

this works:

// example 2
var h
h=$('html')
h.find('body')

This doesn't work:

// example 3
var f
f=$('html').find
f('body')

I get

Error: Permission denied to access property "ownerDocument"

why?

but this works:

// example 4
var a
a = x => $('html').find(x)
a('body')