@-moz-document rule is no longer available from Web content since it could be used by attackers for CSS injection to steal private data in the URL of third-party sites. Firefox users are still able to use this rule in the user stylesheet to personalize their browsing experience.
The at-rule support has already been removed from Nightly and early Beta/DevEdition as of Firefox 59, and removed from all the channels with Firefox 61.
An exception is the empty
url-prefix function that has been used as a CSS hack targeting Firefox. It continues to be parsed on the Release channel to avoid breakages but will be removed in the near future once major compatibility issues are solved.