This gem was designed do make isolated tests more resilient. In isolated tests, a FireMock is no different than a common mock. The only difference is when the test is called on a not-isolated environment. It checks for the presence of the method on the mocked class, and fails if it isn't there. This adds another layer of security for suit tests, without compromising the isolation of unit tests.
It's based on the awesome rspec-fire from Xavier Shay.