@-moz-document rule will no longer be 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 their user stylesheet to personalize the browsing experience.
Update: An earlier draft of this document said the change was made with Firefox 44, but the patch was not merged to that version due to a layout compatibility issue on the
<fieldset> element. The bug is expected to be solved soon.
Update 2: The at-rule support has been removed again from Nightly and early Beta as of Firefox 59. It will soon be completely dropped.
Update 3: Telegram and YouTube are known to be broken due to this change.