browser.cache.disk.enable property should depend on cache filesystem

[Jérôme]

When using a temporary account, the user file directory relies on a memory file system.

In this case, firefox is using the disk cache machinery when it could disable it.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: firefox 45.0+build2-0ubuntu0.14.04.1
ProcVersionSignature: Ubuntu 3.13.0-83.127-generic 3.13.11-ckt35
Uname: Linux 3.13.0-83-generic x86_64
NonfreeKernelModules: nvidia
AddonCompatCheckDisabled: False
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: amd64
AudioDevicesInUse:
 Cannot open /proc/net/unix: Permission denied
                      USER PID ACCESS COMMAND
 /dev/snd/controlC0: guest-eTSQYs 1777 F.... pulseaudio
BuildID: 20160304114926
Channel: Unavailable
CurrentDesktop: XFCE
CurrentDmesg: Error: command ['sh', '-c', 'dmesg | comm -13 --nocheck-order /var/log/dmesg -'] failed with exit code 1: comm: /var/log/dmesg: Permission denied
Date: Thu Mar 31 19:21:40 2016
Extensions: extensions.sqlite corrupt or missing
ForcedLayersAccel: False
IncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
IpRoute:
 default via 192.168.0.254 dev eth1 proto static
 192.168.0.0/24 dev eth1 proto kernel scope link src 192.168.0.10 metric 1
IwConfig:
 lo no wireless extensions.

 eth1 no wireless extensions.
Locales: extensions.sqlite corrupt or missing
Plugins:
 iTunes Application Detector - /usr/lib/mozilla/plugins/librhythmbox-itms-detection-plugin.so (rhythmbox-mozilla)
 Shockwave Flash - /usr/lib/flashplugin-installer/libflashplayer.so
PrefSources: prefs.js
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
Profiles: Profile0 (Default) - LastVersion=45.0/20160304114926
RelatedPackageVersions: rhythmbox-mozilla 3.0.2-0ubuntu2
RfKill: Error: [Errno 2] Aucun fichier ou dossier de ce type: 'rfkill'
RunningIncompatibleAddons: False
SourcePackage: firefox
Themes: extensions.sqlite corrupt or missing
UpgradeStatus: No upgrade log present (probably fresh install)
WifiSyslog:

dmi.bios.date: 03/27/2008
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 0808
dmi.board.asset.tag: To Be Filled By O.E.M.
dmi.board.name: M2N
dmi.board.vendor: ASUSTeK Computer INC.
dmi.board.version: Rev 1.xx
dmi.chassis.asset.tag: Asset-1234567890
dmi.chassis.type: 3
dmi.chassis.vendor: Chassis Manufacture
dmi.chassis.version: Chassis Version
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr0808:bd03/27/2008:svnSystemmanufacturer:pnSystemProductName:pvrSystemVersion:rvnASUSTeKComputerINC.:rnM2N:rvrRev1.xx:cvnChassisManufacture:ct3:cvrChassisVersion:
dmi.product.name: System Product Name
dmi.product.version: System Version
dmi.sys.vendor: System manufacturer