That is something your email provider that you set - Firefox is just a Browser and has no control of email filters. You need to check your online email FAQ for email filtering.

I'm still very confused. Who is my internet provider then? I thought it was Firefox. Does that mean Outlook? or the company that provides my WiFi, Xfinity? I use Windows 10 & Edge & Firefox, that about all that I know.

Hello, in this case, you probably blocked the email address in Outlook. Here are instructions on how to unblock the email address.

If you have more questions, please ask your questions in the Microsoft Support Forum, since this forum is for support with Mozilla products.