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

---
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

{% # Arguments are (in order): `dart`, `libsass`, `node`, `ruby`, optional feature name, additional details within %}
{% compatibility '1.7.2', false, null, 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 %}