Any CSS at-rule that starts with @-moz- is a Gecko-engine-specific rule, not a standard rule. That is, it is a Mozilla-specific extension.
The url-prefix rule applies the contained style rules to any page whose URL starts with it. When used with no URL argument like @-moz-document url-prefix() it applies to ALL pages. That’s effectively a CSS hack used to only target Gecko (Mozilla Firefox). All other browsers will ignore the styles.
See here for a list of other Mozilla-specific extensions.