So given this is a virgin copy of FF with no attachments or anything else other than default settings, why does it exhibit the exact same behaviour?

I have no idea which heading this goes under, but as the title says, when loading a page that needs consent for cookies, the dialog box momentarily flashes up on the screen and is then gone. To add insult to injury I have to fire up another browser, go to the site and it then works, so this is specific to Firefox. I can get this to work if I go into safe mode, but after that I am back to square one. Now if this were any attachments I am using I could understand it, but I recently bought a new Laptop for a friend and installed a fresh copy of FF on it. So given this is a virgin copy of FF with no attachments or anything else other than default settings, why does it exhibit the exact same behaviour?