Depending on the service you are using, go to the blocked senders list and delete or move that entry to the safe list.

Firefox doesn't do email, it's strictly a web browser.

If you are using Firefox to access your mail, you are using "web-mail". You need to seek support from your service provider or a forum for that service.

If your problem is with Mozilla Thunderbird, see this forum for support. https://support.mozillamessaging.com/.../home
or this one
http://forums.mozillazine.org/viewforum.php?f=39