Starting with version 126.0.0, firefox for MacOS stopped creating a defaults/pref folder during installation. If you create it yourself and add the autoconfig.js there, and add firefox.cfg to the root, firefox reads them perfectly and applies the settings. However, if you create folders before the first launch of the browser after installation, the file integrity check fails (see screenshot). I couldn't find any information about why the folders stopped being created. Maybe someone knows? The newest article regarding the configuration seems to be this one: https://support.mozilla.org/en-US/kb/customizing-firefox-using-autoconfig