Firefox 43.0: Downloads fail

I see the same behaviour on Ubuntu 14.04 LTS.

Status changed to 'Confirmed' because the bug affects multiple users.

This bug is funny... Everything worked fine on Sunday. The erratic behavior came back on Monday.

I see the same behavior on Ubuntu Mate 15.10 and Firefox 43.0+build1-0ubuntu0.15.10.1.

I upgraded 2 machines on 19th Dec from 1204LTS to 1404LTS and the Firefox 43.0 which came with the upgrades works fine on these. But a machine which had a bare metal install of 1404LTS a year ago received the Ffx43 update (exactly same build) on Monday and is broken - can't download.
Need to download PDF shipping labels so had to switch to Chrome, which does a much better job of printing these too.

Actually Ffx (per #5) can download PDFs here, it just can't open them in anything other than its own preview. But it won't download and save files.
If I set Prefs > Apps > PDF to Always Ask - it doesn't, and the download history just says 'Failed'. Same if I set it to open in Okular or Adobe Reader. But 'Preview in Firefox' does work OK. Unfortunately Firefox's poor printing doesn't work well enough for the shipping labels so we needed to open them in Okular first and print from there.
This machine also fails to load PDFs from local files and open them in anything other than Firefox Preview - again it shows them under downloads as 'Failed - local file'.

One difference between the 1404LTS machines where PDFs can be opened in other apps (see #5) and the machine where they can't, are the Prefs > Applications options for PDFs:-
PCs where app options work OK list 3 options for PDFs, including 'PDF document (application/x-download) - the relevant one here.
PC where app options don't work (only Preview in Firefox) only offers: 'Portable Document Format (PDF)'.
I tried deleting the mimeTypes.rdf file so it got recreated, and copying it from a machine that works, but neither helped.

I would consider this bug critical. The world is not only PDF documents. My wife uses the Citrix Receiver for home office work. This application relies on configuration files being downloaded and opened by the receiver application. This won't work with the current Firefox, and setting this up in Chromium was a pain.

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).

As a workaround you could try chmod 0777 /tmp/mozilla_mozillaUser0

I experienced the same problem.
@svadim: The workaround "chmod 0777 /tmp/mozilla_mozillaUser0" is working, thank you very much.

I also have the same issue on a large number of PCs.
The workaround mentioned (@svadim - thank you!) does work.
It appears that the /tmp/mozilla_mozillaUser0 dir is owned by the first person to use it with only owner write permissions set & doesn't get removed at logoff or recreated as /tmp/mozilla_mozillaUser# for each additional user.
Setting global write permissions therefore allow any user access to the dir & downloads can function correctly.

¡Hola!

Per https://bugzilla.mozilla.org/show_bug.cgi?id=1233434#c25 this shall be fixed on 43.0.4

From http://packages.ubuntu.com/wily-updates/firefox it is not clear if this is already in Ubuntu.

¡Gracias!