Thunderbird's "maildir-style" support is proprietary in the sense that it's not standard – it's open source, but not like other maildir. It's still file-per-ema

Thunderbird's "maildir-style" support is proprietary in the sense that it's not standard – it's open source, but not like other maildir. It's still file-per-email, but they adapted their implementation to, for starters, support Windows file systems, as the more general maildir format is not supported natively on Windows due to the use of certain characters in the filenames. So, you can use it in Thunderbird, but not share that with other applications that also support maildir, they won't be compatible.