How do I create a new profile to use the data in my old profile?

The normal location would be c:\users\PeterandCarol\appdata\roaming\thunderbird\profiles\sjig3laj.default Knowing exactly what you moved would be useful. The error message you mention doesn't appear to be from Thunderbird. Instead, Thunderbird would more likely post the cryptic, 'no profile found' or similar wording. If you moved ONLY the sjig3laj.default folder you could just move it back.

If I recall, all I did was to move the profile to another location thinking that the spaces in the directory name was actually causing a problem. Unfortunately, trying to repair things seemed to make life worse; if mail storage and all that is under that 'sjig3laj.default' folder, what places to I need to update if I move that folder (and everything under it) to a simpler location? Will the appropriate INI files update themselves?

I finally restored the sjig3laj.default folder to its' original location from backup and all is well now (with the exception of losing a few days' mail ;-( . But when I go into "Personal Settings" I still get that error message telling me "Local directory path ... is not suitable for message storage". Why do I get that, do I care and if so, then how do I make it go away?

The real answer to my performance startup problems would have beed to simply delete sessions.json file so that TB would not try to open files. Oh well, wish I had known that several days ago.

The 'solution' is to COPY the THUNDERBIRD folder itself. That includes the profile and other important elements.