There is no AI here. Despite someone accusing me of being an AI yesterday.

One of the problems with AI is it is only as good as the database it uses to generate it's answers. With something dynamic like the copying of Thunderbird profile (it has changed repeatedly in recent years) the AI is worse than useless because they apparently have no concept of "current"

open Thunderbird on the machine you wish to update. Goto the troubleshooting information on the help menu and click the show profile button in the first block of troubleshooting information.

Close Thunderbird once the windows file manager opens. Now copy you old profile data (that will be in a folder with one of the files being prefs.js) in to replace the files and folders in the profile Thunderbird just opened. You will be asked to replace files. If you are not you are not doing it correctly.

Once the copy is completed restart Thunderbird. You are done.

Alternatively you could use the troubleshooting information on your donor computer to just identify the actual profile folder in use that needs to be copied (including all sub folders. No guessing required this way.

Didn't mean to imply you were AI; I appreciate your help. Found something that had me copy an entire folder named Thunderbird to the desktop. Was a day or two ago, and I'm old, so I don't remember the exact location of the folder on my desktop. I think C:\Users\dalew\AppData\Roaming\Thunderbird and the "clone" on my Desktop seems to work. If that won't work, let me know and I'll follow what ever instructions you have. One thing I noticed about AI, mainly using Copilot, is that the answers it gives are not "up-to-date." It starts by giving "older" data that is not pertinent to today. You end up getting answers that apply to historical app versions. If you keep things up-to-date, as I generally do, you end up getting several messages that don't apply to your situation. I'm slowly adding "as of today" to my questions to get pertinent answers. Usually get better results.