Computer with mandatory user profile, Firefox settings/preferences?

[ShiftF5]

Hello,

I have a series of Windows 7 computers set up with Mandatory profiles some time ago. They are NOT connected to a domain, these are local profiles configured with Local GPO and USER.MAN

Since these computers were set up, their Firefox browsers have updated zillions of times. I wish to change some privacy and security settings in these browsers, however, because of the mandatory profile, any changes I make just get erased when the computer reboots.

I have now set up a Firefox profile that I wish to simply copy the file "prefs.js" into the user's profile, however this gets overwritten upon login as well.

I thought the user's profile was getting copied from either Default or Mandatory.v2 profiles, however neither of those profiles even HAS a Firefox profile folder in them. I also checked All Users and it's not there either.

So where is this profile getting copied from?
C:\Users\username\AppData\Roaming\Mozilla\Firefox\Profiles\<random characters>.default