Firefox 71.0 treats the '#' character (number symbol) as a comment in file
paths.
This means that saving something as /home/user/#file.jpg won't work.
Saving a file to a folder containing '#' doesn't work either, like
/home/user/#folder/file.jpg
Saving as /home/user/test#file.jpg will result in a file called /home/user/test
(no extension).

'#' in the path is fine on Firefox 71.0 on Windows. I don't have any other
Linux distros to test it with unfortunately.

Operating System: openSUSE Tumbleweed 20200105
KDE Plasma Version: 5.17.4
KDE Frameworks Version: 5.65.0
Qt Version: 5.13.1
Kernel Version: 5.3.12-2-default
OS Type: 64-bit
Processors: 24 � AMD Ryzen 9 3900X 12-Core Processor
Memory: 31,3 GiB