First, I located the Thunderbird profile on Windows 10 and copied the entire profile folder to an external hard drive.
Next, I installed Thunderbird on macOS and ran it. It prompted me to create a new account and I canceled that operation. Then, I exited Thunderbird completely.
Now for the "fun" part...locating the "~/Library" directory on macOS. It turns out, the "~/Library" directory (the one in the user's home directory) isn't visible in Finder. To make it visible in Finder, I had to hold the "Option" key while clicking "Go" in the Finder menu bar. By doing that, "Library" appeared as a location and I was able to click it and open it. This information is readily available, provided you think to search for it.
Here's one spot I used as a guide:https://www.macrumors.com/how-to/reveal ... -in-macos/
Once "Library" opened in Finder, I noticed in the "Thunderbird/Profiles" folder there were two profile folders and one ended with "-release". I wasn't sure which one to use to I copied the mail to one, then opened Thunderbird and was prompted to create an account, and then I copied the data to the other one. The profile folder ending in "-release" turned out to be the profile that worked.
When it came to copying the actual mail files, I copied the contents of the profile folder I copied to the external hard drive. I didn't copy the profile file folder itself. The copy operation copied 50+ files, including the actual mail folders that contained the mail.
Once that copy was done, I fired up Thunderbird, on the iMac, and voila! All of the mail accounts appeared, along with messages. Thunderbird prompted me to login to two Google accounts, for the Google calendars that were connected. Thunderbird also started connecting to the email accounts and downloaded new messages. I also conducted a test where I sent some test messages from some of the accounts to my personal email account and all three were received.
I migrated from Thunderbird 91.6.1 on Windows 10 to Thunderbird 91.6.1 on macOS Monterey.
Yippeee!!!!!
Peace...