This is a cursor value that Mozilla used before CSS 2.1
cursor: progress was implemented. Since Gecko 1.7.1 (January 2004), CSS 2.1 cursor progress is supported and rendered properly, so there's no need to use
cursor: -moz-spinning in newer stylesheets.
The -moz-spinning cursor value is used to indicate that the Mozilla browser is performing some processing, but is different from 'wait' in that the user may still interact with the software. The -moz-spinning cursor (just like the progress cursor) may be rendered as a spinning beach ball or as an arrow with a watch or, on the Windows platform, as an arrow with hourglass on its right side.
On the Windows platform, the -moz-spinning cursor value looks like this:
Related bugzilla item: Bug 230343: support cursor: progress