The Problem

Firefox Release 43 has been breaking downloads in multi user systems by creating a folder in /tmp and granting ownership to the first person that does 'Save Link As'

 ls -ld /tmp/mozilla-mozilla* 
 drwx------ 2 user1 user1              4096 Jan  6 10:49 /tmp/mozilla_mozillaUser0
 

Changing the permissions of this file will appear to fix the problem, however this is only temporary, as the next user to download a file will then create a new folder.

 ls -ld /tmp/mozilla-mozilla* 
 drwxrwxrwx 2 user1 user1              4096 Jan  6 10:49 /tmp/mozilla_mozillaUser0 
 drwx------ 2 user2 user2              4096 Jan  6 10:51 /tmp/mozilla_mozillaUser1
 
User2 can download files, but no one else can

On Ubuntu /usr/bin/firefox is a link to /usr/lib/firefox/firefox.sh

 ls -l /usr/bin/firefox 
 lrwxrwxrwx 1 root root 25 Dec 10 10:13 /usr/bin/firefox -> ../lib/firefox/firefox.sh

The Fix

In order to fix this problem effectively, we have added a couple of lines to firefox.sh, near the top:

 ## hack for broken ff43 
 export TMP=/tmp/$USER.firefox 
 mkdir -p $TMP 
 ## hack for broken ff43
So now, in /tmp we see
 ls -ld /tmp/user* 
 drwxrwx--- 4 user1 user1 4096 Jan  6 11:21 /tmp/user1.firefox 
 drwxrwx--- 4 user2 user2 4096 Jan  6 11:24 /tmp/user2.firefox 
 drwxrwx--- 4 user3 user3 4096 Jan  6 11:22 /tmp/user3.firefox 
 drwxrwx--- 4 user4 user4 4096 Jan  6 11:25 /tmp/user4.firefox
All users in the multi user system will be able to download files successfully once the lines of code above have been added and they have restarted firefox, providing a longer term solution than changing file permissions.

Update 15/01/2016

Mozilla have released an update to Firefox 43.0.4 and the multi-user bug appears to have been fixed. On Ubuntu, this also replaced the firefox.sh, thus removing the above temporary fix.

Now in /tmp/ we see

 ls -ld /tmp/moz*
 drwx------ 2 user1  user1  4096 Jan 11 12:09 /tmp/mozilla_user10
 drwx------ 2 user2  user2  4096 Jan 11 09:31 /tmp/mozilla_user20
 drwx------ 2 user3  user3  4096 Jan 11 09:59 /tmp/mozilla_user30
Your distro will probably be pushing through an update to this Firefox version shortly, if it hasn't already.

If you use firefox that you have downloaded and installed manually, then you should update it in the normal way.

Posted by Dominic Mason on 06/01/2016