Yesterday Google announced better integration of Google tasks with iGoogle, mobile devices and Google calendar. This improvement makes Tasks a better app because you don’t need Gmail every time to manage your tasks now.
Unfortunately, there is no official way to use Tasks as a standalone tool like other Google services. :( But, here is hack to open Tasks in a separate browser window : visit this URL https://mail.google.com/tasks/canvas. Now, you can use Tasks as a standalone application. :)
Suggested: Quick ToDo List : Firefox Add-on