_spf.firebasemail.com has no MX records.
Question and response
Question
dig _spf.firebasemail.com. MX
Answer
The response has no answer records.
Authority
firebasemail.com. 300 SOA ns-cloud-c1.googledomains.com. cloud-dns-hostmaster.google.com. 2 21600 3600 1209600 300
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0