The sandbox runs a virtualized copy of browsers. The sandbox folder is meant to be isolated, so when you installed the new Fx version (.13), it most likely failed to remove parts of the last ver. (.12), since it cannot access the sandbox.
Then you try to run .13, and you get the error message you attached, saying you are trying to use .12's XUL runner on .13
As you found out, a simple manual cleaning of sandbox fixes the problem.