Why do think blocking an IP address in Firefox is going to stop email spam from being either sent or received?
Blocking an IP address in your own computer would do nothing to stop spam from being sent from an email account. All you would accomplish is not being able to communicate with your email host. You wouldn't be able to see any of your email messages coming from the blocked IP address, and the spam would continue to be sent and received.

You could set up a spam filter in the email account that is receiving the spam which would direct all email messages received (spoof) email account into the spam folder, and then empty the spam folder on a regular basis.

And of that isn't suitable you should contact the email host for the account that is sending that spam and ask for help or for a solution.

Beyond that this isn't a Firefox support issue, Firefox doesn't do email. Firefox allows webmail services to be accessed and used via web browser protocols HTTP - HTTPS. Email communications use different protocols - POP3 - SMTP.

OK

Thanks