about:home is the default homepage in Firefox, although that 'internal' URI doesn't appear in the Location Bar.

Same security applications on all affected computers? Maybe there's a setting in the security software that prevents preferences from being changed?

Or maybe you have Malware ? ? Strange that Malware would affect multiple devices, unless the same 'carrier' is on all those devices?

A common cause could be in an optional user.js file, a popular place for Malware to place malicious preferences. That would affect Firefox during launch and 'feed' the malicious preference back to the prefs.js file upon startup.

Check your Profile folder for the presence of user.js file. I you got one either delete it or view the contents via a test reader like Notepad or Wordpad to see what it contains.

Yes, same anti-virus software on all machines. This hasn't been a problem until now. People have always set their own homepage preferences. I have checked for malware and there doesn't appear to be any (ran MalwareBytes).

I looked for user.js previously but never see this file. I've even tried deleting prefs.js.

My homepage isn't defaulting back to what you mention in about:home so maybe I've got network policy issues instead of Firefox issues.

Well, something outside of Firefox is probably involved in that, considering that you ruled out having a userjs file in any of those Profiles - the most likely cause inside of Firefox.

Which AV software or Security Suite are you using?

If 'we' know which one you are using maybe someone here will know whether or not it could be causing that to happen - and maybe know what needs to be set to stop that from happpening.

We are using Microsoft System Center Endpoint protection which is essentially Microsoft Security Essentials. I don't think there's any homepage change protection or surfing protection built in.

Thanks for your help!