[regression] default search engines are readded (keywords wiped) with upgrade

Original Bug:
https://bugs.launchpad.net/firefox/+bug/428306

i had detected, that the original search engines are added with an update!

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: firefox 10.0.2+build1-0ubuntu0.11.04.1
ProcVersionSignature: Ubuntu 2.6.38-13.55-generic 2.6.38.8
Uname: Linux 2.6.38-13-generic x86_64
NonfreeKernelModules: fglrx
AddonCompatCheckDisabled: False
AlsaVersion: Advanced Linux Sound Architecture Driver Version 1.0.23.
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC1: frank 2290 F.... xfce4-volumed
 /dev/snd/controlC0: frank 2290 F.... xfce4-volumed
                      frank 2302 F.... pulseaudio
BuildID: 20120216100510
CRDA: Error: [Errno 2] Datei oder Verzeichnis nicht gefunden
Card0.Amixer.info:
 Card hw:0 'SB'/'HDA ATI SB at 0xfe024000 irq 16'
   Mixer name : 'Realtek ALC889A'
   Components : 'HDA:10ec0885,1458a102,00100101'
   Controls : 38
   Simple ctrls : 21
Card1.Amixer.info:
 Card hw:1 'HDMI'/'HDA ATI HDMI at 0xfdffc000 irq 19'
   Mixer name : 'ATI RS690/780 HDMI'
   Components : 'HDA:1002791a,00791a00,00100000'
   Controls : 4
   Simple ctrls : 1
Card1.Amixer.values:
 Simple mixer control 'IEC958',0
   Capabilities: pswitch pswitch-joined penum
   Playback channels: Mono
   Mono: Playback [on]
Channel: release
Date: Mon Feb 20 13:10:15 2012
ForcedLayersAccel: False
IncompatibleExtensions:
 Leo Search - ID={c666c018-6409-4479-afa3-68e4129e7eff}, Version=1.1.0, minVersion=3.5, maxVersion=8.*, Location=app-profile, Type=extension, Active=Yes
 NoScript - ID={73a6fe31-595d-460b-a920-fcc0f8843232}, Version=2.3, minVersion=1.9a2, maxVersion=1.9.6, Location=app-profile, Type=extension, Active=Yes
InstallationMedia: Xubuntu 10.04.2 "Lucid Lynx" - Release amd64 (20110215.1)
IpRoute:
 192.168.0.0/24 dev eth0 proto kernel scope link src 192.168.0.101 metric 1
 169.254.0.0/16 dev eth0 scope link metric 1000
 default via 192.168.0.1 dev eth0 proto static
IwConfig:
 lo no wireless extensions.

 eth0 no wireless extensions.

 vboxnet0 no wireless extensions.
ProcEnviron:
 LANGUAGE=de_DE:en
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
Profiles: Profile0 (Default) - LastVersion=10.0.2/20120216100510 (Running)
RfKill:

RunningIncompatibleAddons: True
SourcePackage: firefox
UpgradeStatus: Upgraded to natty on 2012-01-02 (48 days ago)
dmi.bios.date: 07/08/2010
dmi.bios.vendor: Award Software International, Inc.
dmi.bios.version: F7c
dmi.board.name: GA-MA790GP-UD4H
dmi.board.vendor: Gigabyte Technology Co., Ltd.
dmi.board.version: x.x
dmi.chassis.type: 3
dmi.chassis.vendor: Gigabyte Technology Co., Ltd.
dmi.modalias: dmi:bvnAwardSoftwareInternational,Inc.:bvrF7c:bd07/08/2010:svnGigabyteTechnologyCo.,Ltd.:pnGA-MA790GP-UD4H:pvr:rvnGigabyteTechnologyCo.,Ltd.:rnGA-MA790GP-UD4H:rvrx.x:cvnGigabyteTechnologyCo.,Ltd.:ct3:cvr:
dmi.product.name: GA-MA790GP-UD4H
dmi.sys.vendor: Gigabyte Technology Co., Ltd.