First, try to have the same version installed on both computers. Copy the profile folder from the Windows PC onto a thumb drive. Help/Troubleshooting, Profile Folder, Open Folder to open the profile folder. On the Linux computer, Help/Troubleshooting, Profile Folder, Open Directory, to open the current, active profile. Close TB, delete the contents of this folder (not the folder itself), and copy in the contents (not the folder) of the profile folder on the thumb drive. Delete pkcs11.txt so the saved logins are read on the Linux computer (the file is recreated on restart). Launch TB on Linux, and you should see all your data. You may have to change things like the default apps for opening attachments (or delete handlers.json from the profile to reset the settings).

Very helpful, and thanks!

99% of the way there—everything but the last sentence concerning default apps and handlers.json.  Can you expand a bit on what those do; particularly handlers.json and the effect of deleting it?  Thunderbird now starts and as far as I can see looks absolutely normal.

Edit/Preferences/General/Files & Attachments (TB 91) shows the apps that are defined to open different types of attachments. If you have Windows apps defined there to open e.g. jpg files, those associations probably don't apply in Linux. Deleting handlers.json removes all the entries, and you can re-define them by opening attachments on the Linux computer.

Interesting. Ubuntu and Libre Office are evidently very smart. I did not delete handlers.json, and attachments open correctly anyway—at least for .jpg, .docx and .pdf; only ones I've tried so far. I'll file a mental note so If I hit snags further on I'll remember to go back and revisit this.

Thanks again for the assistance clearing this roadblock.