I have testing this on IE8 and Firefox (latest). I currently cannot use the app at all but managed to use it before in about the same conditions.

on IE8, everytime i want to use the Advanced Import button, it goes to the next page and then never shows the object and fields for me to review before importing.

on Firefox, I do get to that page but when I choose an existing object on the Import to Existing Object option, it freezes firefox and it looks like a memory loop because the firefox.exe process in windows just keeps on growing and growing untill I kill the process.

I have already successfully done the advanced import on the same SFDC org and the same object. However this was only in firefox, not IE.

hope this helps.

Cheers