I want to get a new PC. My emails come in through GMail and are then transferred to Thunderbird. Once transferred they are deleted from GMail. I have several hundred emails in the Inbox of Thunderbird (one year I must delete the majority!) Also some in folders off the Inbox. How do I transfer them over to a new PC? Some are important that I wouldn't want to lose - eg my wife's name!!!!!

Any help would be greatefully received.