OK, I just installed Firefox 4, and am not having any difficulty either. Something else seems to be going on - on your particular internet connection or your particular machine.

 

One thing I did note, and that is that it took a unch of minutes for their downloader to prepare the files. I did not want the installer to be used, as I was just testing the on your behalf - don't want the installer to mess up my mother's machine.

Anyhow, I am wondering if you have some BIG purchased files, and the thing you are seeing is a very large, large delay due to preping all those files for download. I have a big stack of 47 purchases, but all relatively small file sizes  in my queue, and it took quite a long time to get the prep work finished for all of them on my second test. But, if you have purchased any very big files, .....