Firefox is not a e-mail client so the issue is with your webmail service. Check the help service of that application.

(As a side note, you should immediately update your Firefox, as the version you are running is out of date. Update Firefox to the latest release).

Firefox is not an email client or Email provider, so Firefox doesn't block any of your emails. I would check with your email provider (yahoo, gmail, aol, etc.), and also if you use s specific program to check your email (outlook, windows mail, etc.