sass-site/source/documentation/breaking-changes/moz-document.md

title	introduction
Breaking Change: -moz-document	Firefox used to have a @-moz-document rule requiring special parsing. As support is removed from Firefox, Sass is in the process of removing support for parsing them.

Sass has historically supported a special parsing for the @-moz-document rule. As Firefox dropped support for them, Sass will also drop support for the special parsing and will treat it as an unknown at-rule.

There is one exception: an empty url prefix function is still allowed, as that's used in a hack targetting Firefox.

{% codeExample 'moz-document' %} @-moz-document url-prefix() { .error { color: red; } }

@-moz-document url-prefix() .error color: red {% endcodeExample %}

Transition Period

{% compatibility 'dart: "1.7.2"', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

First, we'll emit deprecation warnings for all usages of @-moz-document except for the empty url-prefix hack.

In Dart Sass 2.0, @-moz-document will be treated as an unknown at-rule.