Firefox on Android
Firefox on Android uses the RC4-SHA SSL cipher suite. This must be enabled on your server in order for synchronization to work with Firefox on your smartphone or tablet.
If you use nginx you need to add RC4-SHA to the ssl_ciphers section in your nginx.conf. For more information see the nginx documentation.
If you use apache you need to add RC4-SHA to the SSLCipherSuite section in your apache2.conf. For more information see the apache2 documentation.
Mozilla has a document listing the recommended TLS ciphers.
Firefox Sync on Android does not support SNI. A workaround is to disable it for the domain owncloud is using. Use this in your nginx.conf:
listen 443 default_server ssl;
Additionally, if you are using a self-signed SSL certificate you need to import it to Android via: Settings → Security → Install from storage. Note that Android will only import self-signed certificates with the CA bit set. The import was successful when you see your certificate in Settings → Security → Trusted credentials.