Comment 7 for bug 1796661

It seems Firefox downloads the file to a folder under /tmp and then tries to rename the file to the final location:

Rename operations to a fscrypt folder is not supported from an unencrypted folder, files have to be copied, so this results in an EPERM error code.

18290 rename("/tmp/mozilla_tlb0/FEekjG1c.xlsx.part", "/home/tlb/Downloads/test.xlsx" <unfinished ...>
18247 <... writev resumed> ) = 24
18256 <... futex resumed> ) = 0
18247 poll([{fd=4, events=POLLIN}], 1, -1 <unfinished ...>
18290 <... rename resumed> ) = -1 EPERM (Operation not permitted)

Bind mounting a non-encrypted folder to the same location gives an EXDEV error and Firefox seems to handle that case:

17870 rename("/tmp/mozilla_tlb0/IwoS6IQp.xlsx.part", "/home/tlb/Downloads/test.xlsx" <unfinished ...>
17973 write(38, "\372", 1 <unfinished ...>
17906 <... madvise resumed> ) = 0
17973 <... write resumed> ) = 1
17870 <... rename resumed> ) = -1 EXDEV (Invalid cross-device link)