import email from Thunderbird to MacOS mail app?

Greetings,

I'm looking for any advice on importing mailboxes in macOS Mail app from thunderbird...

i've tried for a long time, and haven't had success.

the macos mail import is unreliably, only some messages show up and many don't. i've tried 1 mailbox file at a time, and very little success.

i've copied the mail folders from TB profile in ~/Library/Thunderbird/...

the file contents look like old fashion mailbox files. e.g, text content with the mail messages concatenated...

macOS mail app creates a mail directory "import #" and has the folder with the name of the mail box imported. but sparce or no content and no error message.

the files mailbox files are perfectly readable in TB and can be viewed in BBEdit...

i can save individual messages from TB and open them in mac mail app. but that obviously looses all the metadata. so not workable...

i can't use the servers to do the transfer, like i'd done sometimes in the past. company outsourced email to microsoft. TB can't connect to usoft. macOS mail app does't connect to the older mail server. so servers aren't an option here.

i have about 14 years of mail in the old server/TB and am really stuck without the mail history and folder structure in macOS mail...

i have read articles around the web and searched apple's forums, but haven't found anything for this problem.

has anyone successfully transfer files from TB to Mac mail app? any utilities that might help?

i'd appreciate any suggests or pointers.

My software that I've been using. : TB 91, then TB 115.3, TB 115.10, now TB 115.12

macOS: 14.5

Mail app: v 16.0 (3774.600.62)

thank you!

jeffery