The simplest way is to copy the Thunderbird folder from the W7 setup to the corresponding location on the W10 computer. The default location is:

C:\Users\{winusername}\AppData\Roaming\Thunderbird

AppData is hidden by default.

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

I understood *that* aspect of the process, using an external hard drive to accomplish the transfer. I regularly access that AppData area on my Windows 7 computer. What concerned me was the "#a###aa.default \Mail" name of the Profile, whether that is somehow built in to the Thunderbird program itself or whether its installed "name" can be changed simply by putting the megabytes of stuff I have into its location. Then too there is the question as to whether all that Mail area includes the email account*s* structural information or whether I would have to start over from scratch REBUILDING each and every one of my email accounts. I have a slew of them for each of numerous aspects of my email life (one of the realities which makes Windows 10 "mail" totally absurd for me to use).

Transferring the Thunderbird folder includes the mail for all accounts and all account settings, for all profiles. The Mail subfolder includes POP accounts mail and Local Folders, but the 'structural information' is mostly stored in the prefs.js file in the profile. The profile name can be changed if desired, using Profile Manager, but the important item is that the profiles.ini file points to the named profiles.

I see that the Prefs.js file is in fact included in the Profile so the copying procedure *is* going to work for getting it transferred over. However, my Windows 7 search engine could *not* find the "profiles.ini" file anywhere so I remain concerned whether the straight forward copying of the ".default" structure into the Windows 10 machine (perhaps renamed to whatever TBird calls it when I install from the web site) is going to work.

The profiles.ini file is in the Thunderbird folder, and it must exist else the program won't read the profile folder. That is why it's recommended to copy the Thunderbird folder between computers, not the ********.default profile folder.

It was only my Windows 7 search engine which couldn't find the "profiles.ini" file. Stepping down through the AppData area on both machines, I found it with no problem. The one discrepancy remaining is that Windows 7 has an original "registry.dat" file which does not "yet" exist on the new Windows 10 downloaded installation. You may have just resolved *that* problem too by indicating that it is the "Thunderbird" folder that needs to be copied over and not only the ".default" folder since within that is where the registry.dat file appears.

Many thanks sfhowes for resolving ALL OF MY QUESTIONS about migrating Thunderbird from Windows 7 to Windows 10. I just copied the "Thunderbird" folder from Windows 7 to replace the automatically generated equivalent on Windows 10 using an external hard drive. Clicked on the brand new today Thunderbird icon on my Windows 10 Desktop and it brought up all of my emails and accounts just as they had been on my Windows 7 machine. I won't be making the actual migration until everything else about the Windows 10 machine has been resolved, but it is very good to know I can continue to have the services of Thunderbird on the new machine when I do switch over.

Incidentally, I bypassed the "donation" thingy to download and install Thunderbird on my Windows 10 machine but given the usefulness of it and Firefox (already using it as replacement for the klunky Windows EDGE on Windows 10 machine) for many years now, I am interested in sending a check to donate to the organization which has been providing such useful software.

Win 10 does not have any such path as "C:\Users\{winusername}\AppData\Roaming\Thunderbird"

@misterr1000 I was puzzled by your response about Windows10 until I realized you might be talking about a computer on which you have NOT installed ClassicShellSetup_4_3_1.exe to create an environment not subject to the absurd abusive noise polluting yammering "helper" sorft WAR which from all appearances (when I originally was working on trying to make Win10 do some actual work) had no conventional PATHS at all. Alternatively perhaps you didn't understand the "{winusername}" part of the path I specified as referring to *your* username as set up on *your* Win10 computer. *With* classic shell installed, it is actually easier to get to the "C:\Users\{winusername}\AppData\Roaming\Thunderbird" than under Windows7 because the subdirectory of {winusername} "Appdata" actually appears in the directory list rather than having to be forced as under Windows7.

misterr1000 said

Win 10 does not have any such path as "C:\Users\{winusername}\AppData\Roaming\Thunderbird"

Press Windows key+R, type %appdata%, OK. AppData is hidden by default.

@sfhowes I gather your instructions for accessing AppData and subsequently the Roaming\Thunderbird directories will somehow work on the "as received" version of Windows 10 with Cortana still operating and that those instructions will facilitate the copying of the crucial "Profiles" directory from an inserted USB disk originating from a Windows 7 computer in place of the original version on Windows 10 that comes with Thunderbird. I hope one means or the other enables @misterr1000 to get Thunderbird running successfully under Windows 10. Incidentally, I did as I suggested I would not only sending a donation to Thunderbird and Firefox not long after my initial success reported here but setting up significantly larger testamentary bequests to each of them. Each of the programs is "best in class" for email client and browser functions.

The %appdata% Run command should work with any version of Windows, but copying the Thunderbird folder, which contains the Profiles folder and profiles.ini, is the recommended process for backup, restore and transfer to another computer.

The TB and Mozilla organizations will surely appreciate your donations.