firefox button not enabling

1    // fix disabled buttons via JS not enabling on page reload /
2    // back navigation.
3    
4    window.addEventListener('pageshow', PageShowHandler, false);
5    window.addEventListener('unload', UnloadHandler, false);
6
7    function PageShowHandler() {
8        window.addEventListener('unload', UnloadHandler, false);
9    }
10
11    function UnloadHandler() {
12        //enable button here
13        window.removeEventListener('unload', UnloadHandler, false);
14    }
15