This is a cursor value that Mozilla used before CSS 3
cursor: context-menu was implemented. Since Gecko 1.8a6 (December 2004), CSS 3 cursor context-menu is supported and rendered properly, so there's no need to use
cursor: -moz-context-menu in newer stylesheets.
The -moz-context-menu cursor value is used to indicate that a context menu is available for the object under the cursor. Often rendered as an arrow with a small menu-like graphic next to it. The -moz-context-menu cursor (just like the context-menu cursor) is rendered as an arrow with a small menu-like graphic on its right side.
At the time of writing this document, the -moz-context-menu is not implemented for the Windows platform and there are no plan to implement it on the Windows platform.
On the Macintosh platform, the -moz-context-menu cursor value can (could?) look like this:
Related bugzilla item: Bug 163174: Support CSS3 cursors