signing.sh

#!/bin/bash
JAR=omni.ja
FILE=modules/AppConstants.jsm

function main() {
  cp -n $JAR $JAR.orig
  zip -F $JAR --out $JAR.fix
  unzip $JAR.fix $FILE
  perl -0777 -i.orig -pe 's/(MOZ_REQUIRE_SIGNING:\n.+?\n\s+)true,/\1false,/' $FILE
  zip -vu0XD $JAR.fix $FILE
  mv $JAR.fix $JAR
}

main

You can verify that the change was applied by running the following in a privileged console
https://developer.mozilla.org/en-US/docs/Tools/Browser_Console

Cu.import("resource://gre/modules/addons/AddonSettings.jsm")
alert(AddonSettings.REQUIRE_SIGNING)

This returns false after the change instead of the default true on release builds. The constant affects the following code paths
https://dxr.mozilla.org/mozilla-central/search?q=MOZ_REQUIRE_SIGNING+path%3Atoolkit%2F&redirect=true