Based on these 2 items
3. It's happening in Most recent Vs. of fire fox.
4. It's only happening on One system which is my client's system.
It is going to be pretty safe to say that something on your client's particular combination of Windows and Firefox is causing the issue.
While safe-mode disables quite a bit, it could still be a particular setting with the installation. You may want to try doing a "Refresh" of Firefox, which will clean up just about everything while retaining bookmarks and passwords.
Refresh Firefox - reset add-ons and settings | Firefox Help[
^]
Another option could be a boinked profile; you can create a new one if those and test as well.
Or maybe there is a virus, malware, or some other booger on the machine.
You have not provided results with other browsers; IE, Edge, Chrome. You did check these, right?