That's possibly related to https://bugzilla.mozilla.org/show_bug.cgi?id=1233434 (Linux: temporary download directory is not created specifically for each user).
** Bug watch added: Mozilla Bugzilla #1233434 https://bugzilla.mozilla.org/show_bug.cgi?id=1233434 -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1527884 Title: Firefox 43.0: Downloads fail To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1527884/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs