Firefox doesn't have any control over what you send or receive from your email. The more likely cause is that your email provider is simply filtering out the emails.

Make sure to check your junk/spam folder in your email. If that doesn't help, contacting your email provider is the best next step for you.

In my experience, some email providers like Yahoo or AOL tend to quite heavily filter out what they consider to be promotional emails.

Hope this helps.

Thanks, but my email provider is MSN, and I don't usually have that problem with it.

In that case, you're best option is likely to contact Microsoft's Outlook.com support team for some assistance.

Since Firefox doesn't have any control over your email, it's unlikely that you will get any helpful advice on this forum.