You can change this in the configuration dialog, and once disabled, that setting will be preserved through Firefox upgrades. You shouldn't need to change this repeatedly.
> legacyUserProfileCustomizations to use userChrome.css to get anything like a proper menu system.
By "proper menu system", do you mean having the menu bar always visible? You can enable that with View->Toolbars->Menu Bar.
> you have to edit policies.json to get updates stopped
Doing so would result in running an outdated and potentially insecure browser.