Techniques are presented for providing seamless extension porting. A second browser of a client device can receive a call to execute extension code containing function calls for a first browser. The extension code can further include a preload manifest key, which can include at least one bridge script property providing a bridge between function calls for the first browser and function calls for the second browser. In response to receiving the call to execute the extension code, the at least one bridge script property of the preload manifest key may be performed to execute the referenced bridge. Subsequently, remaining code of the extension code may then be executed. The bridge can receive function calls by the extension code. When a function call for the first browser is received, the bridge can map the function call for the first browser to a corresponding function call supported by the second browser.