I cannot sync Firefox - it asks for a verification code but the email never arrives.

I'm trying to sync Firefox to my new Mac. It asks me to enter a verification code which it sends to my email - but the email never arrives; I have requested it several times. I have checked Spam, I have checked my webmail as well as on the server, i have designated the Firefox accounts email address a safe sender, i have checked that I am still receiving emails (I am), but still no verification code.