http://kb.mozillazine.org/Move_to_a_new_PC

This post refers to transferring TB on a single computer, before and after a reformat, but the method is the same if you're transferring to another computer:

https://support.mozilla.org/en-US/questions/1105882

Sorry Christ1, the solution you suggested is old and not for Windows10. Plus the person that posted the solution is not longer monitoring it and says to use at your own risk which I'm not willing to do with a new laptop. Thanks anyway.

sfhowes the suggestion you made says to install the copied files into the "Roaming" folder. That might be the problem I am having because the "Thunderbird" help site said to install into the "Thunderbird" folder. I'll try this solution and let you know if this works. Thank you

I found on my first attempt to copy a Thunderbird profile into Windows 10 it simply copied the top level of folders, but not their full contents. I don't know what I did differently, but after two or three further attempts it finally recognized all 19000 files that needed to be copied.

Thanks Zenos I think I'm on about 10 or 12 by now including reinstalling. Took a while to find the "hidden"folders. and now trying a clean re-install. found that it was pulling up old installs.

I hate having to switch computers, but love the new tech. sigh...

Simple, as long as everything copies, and you have not modified the default locations of data files..

On the old computer open troubleshooting information on the help menu. press the show profile button. Close Thunderbird. Press the up button on Windows explorer until your in the appdata folder and can see a Thunderbird folder in the list copy that folder and it's children to a usb drive.

On the new computer, repeat the process. replace the Thunderbird folder on the new device with your copy.

This process assume default locations for account folders and the profile. It also assumes you get all the files in the old profile and they all copy. (Thanks Zenos). Bu copying the Thunderbird folder you get the profiles.ini file that tells Thunderbird where to look for it's files. so as long as you replace one with the other it just works, because there is no need to tell Thunderbird where the files are located.

An alternative is to simply copy the contents of the profile folder and it's sub folder and using troubleshooting information on the new machine, replacing the new profile with your old data. When you do this you should be prompted for lots of replace existing files. If not, it is probably you are pasting your old data one folder to far down the tree.