Copy the Thunderbird folder from the W10 computer to the corresponding location on the OS X computer, then delete the pkcs11.txt file from the profile folder on the Mac. This discussion is for Mac to Windows, but the method is the same.

https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer#thunderbird:win10:tb60

https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer#thunderbird:mac:tb60

my email addresses run on IMAP. Will I face problems if I keep using Thunderbird on Win10 (or Win7) and MacOS at the same time (3 different machines)?

You shouldn't have any problems with IMAP accounts running on multiple computers, phones etc. There are occasional reports of iPhones using too many connections, and sometimes blocking other devices.

I just tried: - I copied the profile folder in .../AppData/Roaming/Thunderbird/ to ~/Library/Thunderbird - I deleted the existing profile folder in Thunderbird MacOS - I deleted pkcs11.txt in the new profile folder

When I try to launch Thunderbird I get the error "Profile missing..."

The error is due to the profiles.ini file in the Mac Thunderbird folder not pointing to the profile folder. You can either edit this file to enter the correct path, or follow the original instructions to copy the Thunderbird folder from the Windows computer, not the profile folder.

yes indeed I misinterpreted the instruction and copied just the profile folder :) Copying the whole Thunderbird dir all works fine.