Why would a test run in NUNIT but not in Visual Studio Unit Tester?

We have a debug project that runs tests designed for N-Unit. When we run the same tests through the Visual Studio debugger (with resharper), it fails.

The code uses selenium. The call where it fails is the call to instantiate a Fire Fox Driver:

driver = new FirefoxDriver(profile);

The exception thrown is curious. It says "{"The system cannot find the file specified"}"