Da bleibt nur, sich in Geduld üben.
Oder folgender (in den Release-Notes zu Tb 52.1.0) mitgeteilter "Workaround":
ZitatLarge attachments may not be shown or saved correctly if the message is stored in an IMAP folder which is not synchronized for offline use. Workaround:
Set preference browser.cache.memory.max_entry_size to the value of -1 (unlimited) and create integer preference browser.cache.memory.capacity with the value of 200000 (200 MB, 25 MB per message).
Gruß
Feuerdrache