FF isn't yahoo so it doesn't have anything to do with Yahoo emails. You should contact Yahoo support about this.

Hi, yes look through your Help on how to block spam or people. Nobody blocks sites that I know of. Put more words in to your spam filters.

If this is coming into your Outlook or some other program like that then buy a program that is a spam blocker/filter.

Please let us know if this solved your issue or if need further assistance.

Even though this isn't a Firefox issue - would you please take a look at this :

https://help.yahoo.com/kb/SLN16025.html

And : https://www.lifewire.com/block-sender-yahoo-1174455

And also : https://www.ghacks.net/2014/01/08/yahoo-mail-filter-block-email-address/

In case you are using Thunderbird : https://support.mozilla.org/en-US/kb/blocking-sender

That should help you block your unwanted emails .....