Mozilla LZ4 (also called jsonlz4 or mozLz4) is a compressed file format that may be used by Mozilla Firefox for bookmark files, and possibly other things. The filename extension "jsonlz4" is used for bookmarks and "mozlz4" for others. It is based on LZ4, and usually decompresses to JSON format. See also Firefox bookmarks.
Files begin with ASCII "
mozLz40", followed by a NUL byte. Next the size of the original file is stored. At offset 12 the compressed data begins.