I thought I'd blog about this, hoping some web developers may find it useful.
Previously, Mozilla's HTTP protocol implementation did not check the presence of the Accept-Ranges response header. The job of this header is to tell the client whether it can perform HTTP range requests, and if so, in what units should it express its range requests. Practically, the only unit used by web servers is bytes, so this header's job in the real world is to tell the server whether it can perform range requests at all.