I uploaded all my files on back up external disk from my old computer and transferred all files and Apps to new mac computer. When I opened the Thunderbird App in the dock to launch I got a message that the application was damaged and tto dump in trash. I now downloaded the new and upgraded Thunderbird Application and trying to install it on my system. I'm an old guy and can do a lot of stuff. I'm sure it is very simple. I just need steps. Thank you. James Tate