Firefox does not respect the application preference order for opening files from the downloads library

[Alvin]

Steps to reproduce

- Check a file association. For example 'pdf'.
  kcmshell5 filetypes
  Application Preference Order is:
    1. Okular
    2. Karbon
    3. ImageMagick (display Q16)
    4. ImageMagick (display Q16)

- Use Firefox to download a pdf file. Firefox preferences for PDF are set to 'always ask'.
- Open the Firefox library at Downloads (down arrow -> Show All Downloads) and double click the downloaded PDF file.
- Chances are it will open using Karbon instead of the expected Okular

Likewise, clicking the folder icon next to the downloaded file will not use Dolphin, but Gwenview and .zip files will not be opened with Ark, but with Gwenview, or something else.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: firefox 39.0+build5-0ubuntu0.15.04.1
ProcVersionSignature: Ubuntu 3.19.0-25.26-generic 3.19.8-ckt2
Uname: Linux 3.19.0-25-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.17.2-0ubuntu1.1
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: alvin 25780 F.... pulseaudio
 /dev/snd/controlC1: alvin 25780 F.... pulseaudio
BuildID: 20150629114049
Channel: Unavailable
CurrentDesktop: KDE
Date: Tue Aug 4 10:15:20 2015
Extensions: extensions.sqlite corrupt or missing
ForcedLayersAccel: False
IfupdownConfig:
 # interfaces(5) file used by ifup(8) and ifdown(8)
 auto lo
 iface lo inet loopback
IncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
InstallationDate: Installed on 2015-04-24 (101 days ago)
InstallationMedia: Kubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
IpRoute:
 default via 10.0.0.2 dev eth0 proto static metric 1024
 10.0.0.0/16 dev eth0 proto kernel scope link src 10.0.3.2
 169.254.0.0/16 dev eth0 scope link metric 1000
 192.168.122.0/24 dev virbr0 proto kernel scope link src 192.168.122.1
Locales: extensions.sqlite corrupt or missing
PrefSources:
 prefs.js
 [Profile]/<email address hidden>/defaults/preferences/prefs.js
 [Profile]/<email address hidden>/defaults/preferences/dnssec.js
Profiles: Profile0 (Default) - LastVersion=39.0/20150629114049 (In use)
RfKill:
 0: hci0: Bluetooth
  Soft blocked: no
  Hard blocked: no
RunningIncompatibleAddons: False
SourcePackage: firefox
Themes: extensions.sqlite corrupt or missing
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 01/08/2015
dmi.bios.vendor: FUJITSU // American Megatrends Inc.
dmi.bios.version: V4.6.5.4 R1.34.0 for D3222-B1x
dmi.board.name: D3222-B1
dmi.board.vendor: FUJITSU
dmi.board.version: S26361-D3222-B1
dmi.chassis.type: 6
dmi.chassis.vendor: FUJITSU
dmi.modalias: dmi:bvnFUJITSU//AmericanMegatrendsInc.:bvrV4.6.5.4R1.34.0forD3222-B1x:bd01/08/2015:svnFUJITSU:pnD3222-B1:pvr:rvnFUJITSU:rnD3222-B1:rvrS26361-D3222-B1:cvnFUJITSU:ct6:cvr:
dmi.product.name: D3222-B1
dmi.sys.vendor: FUJITSU