The simplest way for a 100% transfer is to copy the entire profile. Here are steps for that:

Here is the link for moving a profile in Windows when installed in default location:

https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer

here is the info in brief steps:

1. ensure thunderbird is NOT running on old machine
2. enter %appdata% in search window on task bar and press Enter key
3. this should open windows file explorer in Appdata\roaming folder
4. highlight the Thunderbird folder and copy to external media
5. ensure thunderbird is installed on second computer and NOT running
6. enter %appdata% in search window on task bar and press Enter key
7. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This may prompt to overwrite an existing folder of same name. allow it
8. start thunderbird and all should be there

If you have already setup thunderbird and accounts, you can still get the Local Folders. If you're comfortable working with windows file explorer do this:

- on old hard drive, locate the old profile at the users\<yourid>\appdata\roaming\thunderbird\profiles folder. the profiles.ini file at appdata\roaming\thunderbird will point to the proper profile - open that profile and click to the Mail\Local Folders folder and copy everything there - then, on new computer, paste it all in the Mail\Local Folders folder of the active profile - Do this when thundebird NOT running.

Following your directions to users\<yourid>\appdata\roaming\thunderbird\profiles

There are locations for two mail files on my old computer: One sas a mail file under XXX.default-release that looks like it has data for firefox. The mail folder is this folder has some .sbd folders with 2024 date stamps that contain a file and a .msf file for each corresponding local subfolder folder in the Proton program. Then there are pairs of files and .msf for some of the other local folders all date stamped 2024 under the primary Mail folder.

One mail file under YYY.default, there is no mail folder and just the some .sbd folders (some the same and some different) with 2025 date stamps after June 15, 2025 that contain a file and a .msf file for each corresponding local subfolder folder in the Proton program. Then there are pairs of files and .msf for some of the other local folders that are mostly date stamped 2025. But there is one folder, and .html and a .dat file dated 2020.

There are no folders under the mail Proton account when accessed by web browser

Previously I had used thunderbird on one ISP with one email. June 15, 2025 is about the time, I switched to Proton with split emails because I was losing my ISP because we were moving out the area.

I cleaned out my files from every split email in Proton on the server and reestablished the split emails in Proton on the new computer as well as the old computer.

Any suggestions how to proceed? If seems that I should focus on transferring the mail in newer YYY.default mail file that is more compatible with my new Proton ISP emails.

buyer3, if you copy the thunderbird folder, thunderbird will probably default to the default-release. let me know results.