This is a patch release, which includes bugfixes for some critical bugs. It is meant to be backwards compatible to qooxdoo 1.2. Nothing needs to be changed in the JavaScript code of your existing apps, if they are based on the previous 1.2 release.
Bug 4058: qooxdoo apps no longer break in Firefox 4 Beta. This was due to the removal of XUL labels in Firefox, which were used to display a text ellipsis for labels. Unless Firefox ships with an equivalent feature, text ellipsis won't be available.
Bug 4019: qooxdoo apps no longer break in IE9 Beta in standards mode. When creating DOM elements, the browser is now compatible to the DOM specification.
Bug 4045: Applications built under Windows now use combined images as expected. It was a regression that only affected release 1.2.