I had the same problem. I searched MTP support and found a link to Microsoft download center and downloaded a Media Transfer Protocol Porting Kit. I also updated Media center to the newest one. The Porting kit changed the way the Kindle shows in "my computer" and now I can just open it to the book folder and it lets me copy my mobi files with no problem. Just Google MTP support and the MS link will come up.
I still use Calibre for my regular Kindle and my daughter's Nook, it's a great program. You can still use it to convert files to mobi for the Kindle fire.
Last edited by Cheyanne; 12-27-2012 at 06:27 PM.