Enumerable cross-origin properties don't seem to be web-compatible

Updating Firefox to #2777 caused a web compat issue: if a cross-origin window makes its way into jQuery.extend, that function will go into an infinite loop. This is turning up as a problem on actual sites. See https://bugzilla.mozilla.org/show_bug.cgi?id=1412741 for details.

I will be backing out the corresponding changes from Firefox.