window.onunload is not working as expected.

My Scenario . On Close of the browser/ tab / refresh the tab we need to execute the piece of code. All my code is in window.onbeforeunload which used to work in previous version of firefox . But with the latest version code is executing only on closing the tab but not in the browser close or refresh . So i kept return statement in the window.onbeforeunload and all the code in the window.onunload . Now on click on the Leave this page it should call window.onunload and execute the code but in firefox it is not working like this. can you please provide me work around or solution for this. Thanks alot.