I discovered this today because it is a fresh install and it was my first
attempt at downloading something via the browser (a pdf file).

I did al the usual troubleshooting stuff for FF and nothing worked. I
tried SeaMonkey and that worked fine. Then I tried my spare account and
that also worked.

Searching on google for some hints and I found following two bug reports:

1. Firefox 43.0: Downloads fail
https://bugs.launchpad.net/ubuntu/+s...x/+bug/1527884

2.. https://bugzilla.mozilla.org/show_bug.cgi?id=1233434
Linux: temporary download directory is not created specifically for each
user

The workaround fix I got for myself is by deleting
'mozilla_mozillaUser0' in the /tmp directory and then launching FFox
in the main account to recreate it and lock it in as its own. That will
do me for now and I should hope a patch to fix this issue will be
forthcoming soon via the updates.

Lastly, I apologise if it has been covered already but I could not
see anything in the Google search results.

Cheers,
Peter