I do not know OSX, but here are the basic steps:

Here is the link for moving a profile in Windows when installed in default location:

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

here is the info in brief steps:

1. ensure thunderbird is NOT running on old machine
2. enter %appdata% in search window on task bar and press Enter key
3. this should open windows file explorer in Appdata\roaming folder
4. highlight the Thunderbird folder and copy to external media
5. ensure thunderbird is installed on second computer and NOT running
6. enter %appdata% in search window on task bar and press Enter key
7. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This may prompt to overwrite an existing folder of same name. allow it
8. start thunderbird and all should be there

Hi David, I've followed exactly those steps.

the main issue is that when I start thunderbird on windows 11, it just starts as if it's new and doesn't see any of the old stuff.

Thunderbird sometimes does that for a new install. Try this: - click help>troubleshootinginformation - scroll down to 'profiles' and click 'about:profiles' - your old profile should be there. see if you can launch it.