That's odd because mobi files are perfect for Kindles. They're the ones I always try to find when I'm downloading fanfic and ebooks. Hm. Strange. :( I'd try downloading another mobi file and seeing if it works because I've never had a problem with them, but I just have a regular Kindle 3G so I don't know if the Fire is different in regards to mobi files or not.

Oh good! LOL. I always hate butting into convos, I just wish I could've been more helpful. ;)