kill_all.py

    def kill_all(self):
            self.marionette.switch_to_frame()
            self.marionette.execute_async_script("""
                // Kills all running apps, except the homescreen.
                function killAll() {
                let originsToClose = [];
                let that = this;

                let runningApps = window.wrappedJSObject.AppWindowManager.getRunningApps();
                for (let origin in runningApps) {
                if (origin.indexOf('homescreen') == -1) {
                window.wrappedJSObject.AppWindowManager.kill(origin);
                }
                }

                if (!originsToClose.length) {
                marionetteScriptFinished(true);
                return;
                }

                originsToClose.slice(0).forEach(function(origin) {
                kill(origin, function() {});
                });
                };
                killAll();
            """)