Looks like pcap decryption works only with .pem files. But TLS decryption with NSS isn't available (https://www.openssl.org/docs/man1.1.1/man3/SSL_CTX_set_keylog_callback.html, https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/Key_Log_Format, https://sharkfesteurope.wireshark.org/assets/presentations17eu/15.pdf)