This is not a Firefox problem but rather a problem with your e-mail provider. Maybe the e-mails are being flagged as "spam" or "junk" mail? That happened to me recently..... with e-mail notifications from support.mozilla.org! I discovered the problem by going to the web-based e-mail portal for my mail provider and checking my SPAM folder. Sure enough, they were being flagged as spam so I had to adjust my account settings for filtering spam to a lower setting. Anyway, ask your e-mail provider.