moz_hash.py reimplements the getVerificationHash() method from Mozilla SearchUtils.jsm. fix_search_hash.py uses that to modify the search engine entry.

fix_search_hash.py

#!/usr/bin/env python3

#
# Adjusts the metaData.hash value in the supplied search.json.mozlz4 file to match the supplied profile path value.
#

import io
import json
import mozlz4a
import os
import moz_hash
import sys

if __name__ == "__main__":
    inputFile = sys.argv[1]
    newSearchEngineName = sys.argv[2]

    with open(inputFile, "rb") as inFp:
        data = mozlz4a.decompress(inFp)

    jsonData = json.loads(data.decode("utf8"))

    #print("old hash='%s'" % jsonData["metaData"]["hash"])
    hashStr = moz_hash.getVerificationHash(os.path.dirname(inputFile), newSearchEngineName)
    #print("new hash='%s'" % hashStr)

    jsonData["metaData"]["hash"] = hashStr
    jsonData["metaData"]["current"] = newSearchEngineName

    newJsonData = json.dumps(jsonData)
    newCompressedData = mozlz4a.compress(io.BytesIO(newJsonData.encode("utf8")))
    with open(inputFile, "wb") as outFp:
        outFp.write(newCompressedData)

moz_hash.py

#!/usr/bin/env python3

#
# Reimplements the getVerificationHash() method from https://hg.mozilla.org/mozilla-central/file/tip/toolkit/components/search/SearchUtils.jsm
#

import base64
import hashlib
import os
import sys

def getVerificationHash(profilePath, nameParam):
    # the static disclaimer that is added to the hash
    disclaimer = \
      "By modifying this file, I agree that I am doing so " + \
      "only within $appName itself, using official, user-driven search " + \
      "engine selection processes, and in a way which does not circumvent " + \
      "user consent. I acknowledge that any attempt to change this file " + \
      "from outside of $appName is a malicious act, and will be responded " + \
      "to accordingly.";
    services_appinfo_name = "Firefox" # some magic value (Services.appinfo.name in Firefox code)

    salt = os.path.basename(profilePath.rstrip("/")) + \
        nameParam + \
        disclaimer.replace("$appName", services_appinfo_name)
    data = salt.encode("utf8")

    hasher = hashlib.sha256()
    hasher.update(data)

    hash = hasher.digest()
    return base64.encodebytes(hash).strip().decode("ascii")

if __name__ == "__main__":
    profilePath = sys.argv[1] # path where profile is stored (this is used so that the hash is different for every profile)
    nameParam = sys.argv[2] # the "payload" that shall be hashed

    print(getVerificationHash(profilePath, nameParam))