One of the cool features that the recent March CTP drop of Atlas enables is drag/drop support with FireFox for ASP.NET 2.0 WebParts. This doesn't require any coding changes -- all you need to-do is update your web.config file with a few Atlas entries, and then you'll be able to have drag/drop for both IE and FireFox.
David Barkol has posted a nice blog entry describing how to enable it. You can read about it here. Below is an example of it in action with FireFox:
Hope this helps,