If browsing history has been deleted from Firefox (using Clear Private Data or by clearing data in the Android Application Management dialog) then as far as I know, it is not recoverable from the device. If the browsing was done through a wifi hotspot, the router might have some record of what sites were accessed, depending on its features. The internet service provider probably has a record, but may not be willing to share the data without a court order.

I'm not sure what you mean about another account being created. An account on the device, or an account on a website??