Which Fire?
We gave the son a HD8 (w/ads) for Xmas.
Current calibre would not see it. W10 saw it as a MTP device and I used drag and drop to drop 2 AZW3 books into
'Books' on the SD card.
Opened the Kindle app. No books. Poked around the settings. 1 book shows.

Gave up for the night.
Next day, replaced the books with Mobi + a new one. 1 Book. Rebooted the HD8. 1 book. Watched a vid. Went back to the KLibrary. ALL books

(calibre still does not recognize)