cannot add plug-in to Firefox

[Humphrey van Polanen Petel]

When trying to install a search plug-in (https://addons.mozilla.org/en-US/firefox/addon/wiktionary-en/?src=ss), Firefox reports "Sorry, you need a Mozilla-based browser (such as Firefox) to install a search plugin."

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: firefox 40.0.3+build1-0ubuntu0.14.04.1
ProcVersionSignature: Ubuntu 3.16.0-49.65~14.04.1-generic 3.16.7-ckt15
Uname: Linux 3.16.0-49-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.14.1-0ubuntu3.13
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: humphrey 1581 F.... pulseaudio
 /dev/snd/controlC1: humphrey 1581 F.... pulseaudio
BuildID: 20150826185918
Channel: Unavailable
CurrentDesktop: Unity
Date: Sat Sep 19 14:07:02 2015
ExecutablePath: /usr/lib/firefox/firefox
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 2015-09-08 (11 days ago)
InstallationMedia: Ubuntu 14.04.2 LTS "Trusty Tahr" - Release amd64 (20150218.1)
IpRoute:
 default via 192.168.0.1 dev eth0 proto static
 192.168.0.0/24 dev eth0 proto kernel scope link src 192.168.0.101 metric 1
IwConfig:
 eth0 no wireless extensions.

 lo no wireless extensions.
Locales: extensions.sqlite corrupt or missing
PrefSources:
 prefs.js
 [Profile]/<email address hidden>/defaults/preferences/prefs.js
ProcEnviron:
 LANGUAGE=en_AU:en
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_AU.UTF-8
 SHELL=/bin/bash
Profiles: Profile0 (Default) - LastVersion=40.0.3/20150826185918 (In use)
RelatedPackageVersions:
 totem-mozilla 3.10.1-1ubuntu4
 rhythmbox-mozilla 3.0.2-0ubuntu2
RfKill:

RunningIncompatibleAddons: False
SourcePackage: firefox
Themes: extensions.sqlite corrupt or missing
UpgradeStatus: No upgrade log present (probably fresh install)
WifiSyslog:
 Sep 19 09:37:08 David kernel: [ 3659.488030] usb 1-5: new high-speed USB device number 3 using ehci-pci
 Sep 19 09:37:08 David kernel: [ 3659.620347] usb 1-5: New USB device found, idVendor=0424, idProduct=2514
 Sep 19 09:37:08 David kernel: [ 3659.620351] usb 1-5: New USB device strings: Mfr=0, Product=0, SerialNumber=0
 Sep 19 09:37:08 David kernel: [ 3659.620649] hub 1-5:1.0: USB hub found
 Sep 19 09:37:08 David kernel: [ 3659.620716] hub 1-5:1.0: 4 ports detected
dmi.bios.date: 04/17/2009
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 1.1.4
dmi.board.name: 0N185P
dmi.board.vendor: Dell Inc.
dmi.board.version: A02
dmi.chassis.type: 3
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvr1.1.4:bd04/17/2009:svnDellInc.:pnVostro420Series:pvr:rvnDellInc.:rn0N185P:rvrA02:cvnDellInc.:ct3:cvr:
dmi.product.name: Vostro 420 Series
dmi.sys.vendor: Dell Inc.