I found this, might better help shed light on why 4chan does not support JPGs encoded with mozjpg.
https://dev.exiv2.org/projects/exiv2/wiki/The_Metadata_in_JPEG_files
Normal huffman table:
FFC4 001F 00
00 01 05 01 01 01 01 01 01 00 00 00 00 00 00 00
00 01 02 03 04 05 06 07 08 09 0A 0B
Optimized huffman table:
FFC4 001B 00
00 03 01 01 01 01 01 01 00 00 00 00 00 00 00 00
00 01 02 03 04 05 06 07 08 09
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