One regression test selection technique proposed for object-oriented programs is the Class firewall regression test selection technique. The selection technique selects test cases for regression test, which test changed classes and classes depending on changed classes. However, in empirical studies of the application of the technique, we observed that another technique found the same defects, selected fewer tests and required a simpler, less costly, analysis. The technique, which we refer to as the Change-based regression test selection technique, is basically the Class firewall technique, but with the class firewall removed. In this paper we formulate a hypothesis stating that these empirical observations are not incidental, but an inherent property of the Class firewall technique. We prove that the hypothesis holds for Java in a stable testing environment, and conclude that the effectiveness of the Class firewall regression testing technique can be improved without sacrificing the defect detection capability of the technique, by removing the class firewall.