One of the things to look for when changing email clients is what the new application offers in terms of its ability to import data from other clients. I have never used Apple Mail so I can't comment on this.

If it offers no help then the next step might be to look for tools (usually at a cost) which can import or convert mailbox files from one system into another.

Finally you can export messages from Thunderbird, using a format that the new client can use. The eml file is Thunderbird's default export format. You'll need to find out if the Apple Mail program can use these. The major snag is that you can't preserve the folder system you have been using, so you might have to proceed folder by folder, keeping track of names and the folder system structure.

If all else fails you can export the messages to some form of stand-alone format (such as html or pdf) that doesn't need an email client for you to be able to read it. Again, folder structure may be lost, and you won't easily be able to use the text of older messages in new messages. Some clumsy copy-and-paste via your clipboard would be called for.

thanks they all looked like good solutions, but I couldn't find how to export emails from Thunderbird in any format.

Select some messages, go to File|Save As, or right-click and select Save As. This is built in.

For more ambitious exporting (and importing) tasks, look at the ImportExportTools add-on.

https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

The Save as would have been a blessing but the import/export add-on did exactly what I needed. Thanks