Firefox pdf plugin not remembering download directory and moving files from /run/user/XXX deletes them without a trace

[Josef Wolf]

When a pdf file is previewed in foirefox, the pdf previewer offers a button to
save the file. In older installs (e.g. ubuntu-18.04, maybe prior to snap?) the
dialog activated by this button remembered the directory that was selscted in
previos invocations. It even remembered the directory dependant on the site
the file was downloaded from.

In ubuntu-22.04, this dialog always resets to something like
/run/user/1000/XXXX/YYYY, so one has always to manually navigate to the last
download directory that was used last time.

Even worse: when I noticed this fact, I ran:

  $ cd /some/dir/very/deep/into/the/file/hierarchy
  $ find /run/user/1000 -name '*.pdf' -print0 | xargs -0 mv -t .

to move those files to the directory they belong.

With this command, I expected mv(1) to:

- EITHER complain that mv(1) is not designed to move files accross
  filesystem boundaries (this was the behaviour of mv(1) on very very very old
  unixes, back in early 1980 or something)

- OR delegate to cp+rm, which is behaviour of mv(1) since decades.

Contrary to my expectation, the files simply VANISHED without any warning or notice!

IMHO, this behaviour is simply not acceptable. While not remembering download
directory is just a inconvence, moving files to /dev/null is definitely not
the way to go!

-- 
Josef Wolf
jw at raven.inka.de