What email provider are you referring to? FF isn't a email software.

If you are using Thunderbird please post your questions here : https://support.mozilla.org/en-US/products/thunderbird

Thanks for trying but I still need to know why and how to stop it from happening.

Alloyd said

Thanks for trying but I still need to know why and how to stop it from happening.

That would be great to help you but with out information, we can not help you at all. Please answer the questions or go to Thunderbird Support if it is that product.