FireWire to Thunderbolt Adapter
You can use a DisplayLink adapter to connect another monitor via USB; see here for the varieties available.

The Apple Thunderbolt to FireWire adapter only works in that direction; it will not convert FireWire to Thunderbolt. 

You cannot modify your mini to have another Thunderbolt port. You can get a Thunderbolt dock that will give you a monitor port and another Thunderbolt port on the dock, which might also work, but is more expensive than DisplayLink (make sure to get a dock that will "daisy-chain".