Mangles css function definitions (@-moz-document url-prefix())

[UlrichKu]

If a definition like this is used in a css file.
Then there will be a closing bracket } missing in the minified output.

@-moz-document url-prefix() {
  .btn { margin: 0 }
}
@media (max-width: 768px) {
  @-moz-document url-prefix() {
    .btn { margin: 5px; }
  }
}

A workaround is to separate the two blocks by another (unrelated) css definition.

Also note: As far as I understand it this kind of functionality will be removed from Firefox