from your website:
Amazon have removed the ability to manipulate collections completely in their newer models, like the Kindle Touch and Kindle Fire, making even the above plugin useless. If you really want the ability to manage collections on your Kindle via a USB connection, we encourage you to complain to Amazon about it, or get a reader where this is supported, like the SONY Readers.
This doesn't say it doesn't recognize Kindle Fire. I am confused.