Try adding accounts@firefox.com to your contact list or address box. Certain email providers have really strict filters that sometimes block messages. Adding it to your list of trusted senders should prevent it from being blocked.

Also, if you have just requested the email, it might take some time to actually come through. Certain email providers are slower and put messages through a more intensive filter process, which make take longer.

accounts @firefox.com is already in my address book/contacts list. I use Outlook.com for my emails and no email is received, even upon checking my spam/junk mailbox.