I'm sorry to hear about the trouble you're facing with Thunderbird. It sounds like your emails might not have been carried over during the upgrade. Sometimes, during updates, issues like this can occur. First, let's try not to worry too much – your emails might not be lost permanently. Here's what you can do: In Thunderbird, go to the "Help" menu and select "Troubleshooting Information." In the new tab that opens, click on the "Open Folder" button next to "Profile Folder." This will open a new window with your Thunderbird data. Look for a folder named "Old Thunderbird Data" or something similar. Inside that folder, you might find your missing emails. You can then copy them over to your current Thunderbird profile folder. If you can't find them there, don't worry, there are other steps we can try. But give this a shot first. Remember, I'm here to help, so if you have any questions or need further assistance, feel free to ask. Good luck!