Hi,

I had this problem too and it's gone now... I'm not 1oo% sure why, but going by what I've change about my PC and firefox since then, updating Firefox, Java and Flash to the latest version might help. And check your PC's date and time settings

If this doesn't work, could you tell me which e-mail provider you're using... Hotmail/Outlook, Gmail, Yahoo, or something else?

Oops, you have a duplicate thread with additional responses here: https://support.mozilla.org/questions/955945