<===> input.scss
// An empty url-prefix() should not be deprecated yet, as it's still supported
// in Firefox's release channel at time of writing.

@-moz-document url-prefix() {
  a {b: c}
}

@-moz-document url-prefix("") {
  a {b: c}
}

@-moz-document url-prefix('') {
  a {b: c}
}

<===> output.css
@-moz-document url-prefix() {
  a {
    b: c;
  }
}
@-moz-document url-prefix("") {
  a {
    b: c;
  }
}
@-moz-document url-prefix("") {
  a {
    b: c;
  }
}

<===> output-libsass.css
@-moz-document url-prefix() {
  a {
    b: c;
  }
}

@-moz-document url-prefix("") {
  a {
    b: c;
  }
}

@-moz-document url-prefix('') {
  a {
    b: c;
  }
}