the most "fun" I had it with the mess recently was after icedove -> thunderbird name migration (it was renamed in Debian coz of some trademark issue).
Tunderbird, of course, kept mail and config in same directory. The migration process renamed .icedove to .thunderbird and removed it.
But the config for mailboxe still had .icedove/xyz path. So once config migration is finished, mails start to download in old location, and after restarting thunderbird goes "hey, there is both .icedove and .thunderbird dir, I'm not starting".
with XDG that config would be separate so there would be no need to even move the data dir
Tunderbird, of course, kept mail and config in same directory. The migration process renamed .icedove to .thunderbird and removed it.
But the config for mailboxe still had .icedove/xyz path. So once config migration is finished, mails start to download in old location, and after restarting thunderbird goes "hey, there is both .icedove and .thunderbird dir, I'm not starting".
with XDG that config would be separate so there would be no need to even move the data dir