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There seems to be various JavaScript+browser specific ways of decompressing this, but isn't there some way to transform jsonlz4 files to something unlz4 will read?

I was able to unpack the jsonlz4 by using lz4json:

apt-get install liblz4-dev

git clone https://github.com/andikleen/lz4json.git

cd lz4json

make

./lz4jsoncat ~/.mozilla/firefox/*/bookmarkbackups/*.jsonlz4

Save this script as e.g. mozlz4:

#!/usr/bin/env python

from sys import *

import os

try:

    import lz4.block as lz4

except ImportError:

    import lz4



stdin = os.fdopen(stdin.fileno(), 'rb')

stdout = os.fdopen(stdout.fileno(), 'wb')



if argv[1:] == ['-c']:

    stdout.write(b'mozLz40' + lz4.compress(stdin.read()))

elif argv[1:] == ['-d']:

    assert stdin.read(8) == b'mozLz40'

    stdout.write(lz4.decompress(stdin.read()))

else:

    stderr.write('Usage: %s -c|-d <infile >outfilen' % argv[0])

    stderr.write('Compress or decompress Mozilla-flavor LZ4 files.n')

    exit(1)

Usage:

mozlz4 -d <previous.jsonlz4 >previous.json

mozlz4 -c <previous.json >previous.jsonlz4

Actually almost all Firefox profile lz4 files are mozlz4 files. It means they have the same "file format header".
Except one file. I talk about webext.sc.lz4 file. It has mozJSSCLz40v001 file header and maybe some sc packaging to pack group of files to on byte stream.

There is a Firefox addon to read or compress .mozlz4 text files mozlz4-edit

Sufficiently persistent Googling for this turns up a lot of solutions, but most of them seem to be either (a) broken by subsequent changes to underlying libraries, or (b) unnecessarily complex (at least to my personal taste), making them clunky to drop into existing code.

The following appears to work at least on Python 2.7 and 3.6 using a recent version of the Python LZ4 bindings:

def mozlz4_to_text(filepath):

    # Given the path to a "mozlz4", "jsonlz4", "baklz4" etc. file, 

    # return the uncompressed text.

    import lz4.block

    bytestream = open(filepath, "rb")

    bytestream.read(8)  # skip past the b"mozLz40" header

    valid_bytes = bytestream.read()

    text = lz4.block.decompress(valid_bytes)

    return text

Of course this does not attempt to validate inputs (or outputs), is not intended to be secure, etc., but if one just wants to be able to parse one's own FF data, it gets the basic job done.

Command line version here, which could be saved in the relevant directory and invoked from the command line as:

chmod +x mozlz4.py

./mozlz4.py <file you want to read> <file to save output to>