Instead of the usual 16 byte length for AES cipher, AES cipher with KW mode (KeyWrap) processes data in 8 byte blocks, thus the padding should use 8 instead of 16 as the block size when padding data. In addition, NSS's KWP impl ignores the specified IV and always use default IV. Thus, for max interoperability, it's probably safest to stick with default IV for KWP mode, at least for now.