FireFox is not working proper with URL.revokeObjectURL

I am still getting into JavaScript and got a problem I do not understand.

The Code:

function loadDocumentFromResponse(requestObject, filename){
        console.log('Response from Server gets analyzed...');                                           
        if( typeof URL === "function" ){                                                                
                console.log('Will use the URL function to revoke an object url');
                var url = URL.createObjectURL(requestObject.response);
        var a = document.createElement('a');
        a.setAttribute('href', url);
        a.setAttribute('download', filename);
        a.click();
        URL.revokeObjectURL(url);
        // is working with chrome and gets me a download which works
        // firefox also gets into this tree but it does nothing!
        }else{
                // IE, Maxton, Safari and Opera are throwing an Exception, will figure out whats wrong, later....
                console.log('Function URL ist unbekannt');
        }
}

My problem is actually the IF case... Where chrome is exactly doing what I expect, it gives me a download. But Firefox, gots the function URL. too and does not create a download from the blob response...

Whats my fault? And any hint for IE, etc.?

cu