Widget Signout error ... Firefox only


I’m experiencing a strange behaviour with the okta widget and the following code :

SignOut: function () {
var oktaSignIn = new OktaSignIn(this.GetWidgetConfig());

    oktaSignIn.session.close(function (err) {
        // The user has not been logged out, perform some error handling here.
        if (err) {
            console.log('Error at SignOut: ' + err);
        console.log('Session closed');

Everything works well with IE, Edge, Chrome.
But i’m always redirected to the err function with FireFox.

Has anyone ever experienced this before ? I’m stuck …
Thanks for your feedback,