#test {
  width: 100px;
  height: 100px;
}

.enabled {
  background: red;
}

.disabled {
  background: green;
}

@supports -moz-bool-pref("testing.supports.moz-bool-pref") {
  .enabled {
    background: green;
  }

  .disabled {
    background: red;
  }
}