Thank you for your reply. I did some internet search and found that many other snaps are also affected by this “design”, most notably Firefox.
There they also had endless debates about $DOWNLOAD/firefox.tmp. It took nearly a year that they finally fixed it somehow and removed exactly the same line as in Thunderbird’s bin/tmpdir:
https://bugzilla.mozilla.org/show_bug.cgi?id=1733750 https://github.com/canonical/firefox-snap/commit/250d24a72953fd797a10ec2fe338fcd0fe2491ff
So I won’t go into this one and I won’t start yet another fruitless discussion about the fundamental difference between a temporary directory and a download directory. And I definitely don’t want to pollute my home directory or any sub-directory (and it’s backups!) with piles of temporary files left behind by some sloppy designed snaps.
Additionally this snap migration also missed to properly move my Thunderbird profile directory correctly, which also took me some time to research and fix. That’s too much for me, so I will simply avoid snap in the future.