I have just installed Norton 360 and followed their steps to become compatible with Thunderbird but still cannot receive emails

I have just installed Norton 360 but am not able to receive emails. I followed all the steps as outlined in the Norton help site as much as possible and Thunderbird tells me that wscert.der is installed. However it was all to no avail. When doing so I did not see the screen asking me to trust this CA to identify web sites but every other step seemed OK.