There are generally three methods to block Fire OS automatic upgrade. The first method requires Root, the second requires FireOS to be lower than a certain version, and the third method is to truncate the HTTPS request of the Fire OS automatic upgrade service.
The first and second methods are not very easy to operate, and the third method is relatively simple. You can intercept the communication between Fire OS and the following servers.
d1s31zyz7dcc2d.cloudfront.net
amzdigital-a.akamaihd.net
amzdigitaldownloads.edgesuite.net
softwareupdates.amazon.com
updates.amazon.com