This is a cursor value that Mozilla used before CSS 3
cursor: copy was implemented. Since Gecko 1.8a6 (December 2004), CSS 3 cursor copy is supported and rendered properly, so there's no need to use
cursor: -moz-copy in newer stylesheets.
The -moz-copy cursor value is used to indicate something is to be copied. The -moz-copy cursor (just like the copy cursor) is rendered as an arrow with a small plus sign at its top-right corner.
On the Windows platform, the -moz-copy cursor value can (could?) look like this:
Related bugzilla item: Bug 163174: Support CSS3 cursors