Migrating Local Folders from Thunderbird to Mail.app
All your Thunderbird data, including the local mail data is stored in the Profile folder, located either at ~/Library/Thunderbird/Profiles/&lt;Profile name&gt;/, or "~/Library/Application Support/Thunderbird/Profiles/&lt;Profile name&gt;/.

Deleting the Thunderbird app won't delete any of the data there, unless you use a 3rd party uninstalled app like AppZapper. You can of course make a manual backup of those folders for added safety.