Why might Amazon login result in the error "SSL_ERROR_RX_RECORD_TOO_LONG"

You are connecting to https://www.amazon.com:80/. Your browser expects a SSL handshake to take place because of the https protocol part. But the website actually serves a plain HTTP response because you also explicitly specified port 80, the default port for unencrypted HTTP.

Firefox is therefore misinterpreting the HTTP response as malformed SSL traffic which only accidentally looks like a "too long RX record". The error should be gone if you type in https://www.amazon.com/ or https://www.amazon.com:443/. (Port 443 is the default for HTTPS.)

You can provoke similar behavior with many other sites. Here is how Chrome similarly yields a ERR_SSL_PROTOCOL_ERROR when visiting https://google.com:80/: