---
title: 'Breaking Change: -moz-document'
introduction: >
  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.
---

{% markdown %}
  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.

  [Firefox dropped support for them]: https://web.archive.org/web/20200528221656/https://www.fxsitecompat.dev/en-CA/docs/2018/moz-document-support-has-been-dropped-except-for-empty-url-prefix/

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

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

{% markdown %}
  ## 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.
{% endmarkdown %}