Bug 772153 added support for unprefixed cursor: zoom-in | zoom-out (Fx24).
Support for -moz-prefixes should be dropped after
Unprefixed grab | grabbing are implemented in Bug 880672 (Firefox 27).
Besides, shouldn't bugs 772153 and 880672 be blocking this issue rather than is blocked by this?
Lastly, most web authors supply a fallback image or unprefixed zoom-in | zoom-out | grab | grabbing as a value for the cursor, so I think "<inset-arbitrary-number-here>" does not need to be a big number.
Posted the site compatibility doc: https://www.fxsitecompat.com/en-US/docs/2015/prefixed-cursor-types-will-be-removed/