sorting of windows is upside down after private mode

[LAZA]

i have some windows opened up and change sometime into the private mode
--> all windows on the taskbar where closed and a new (in private modus) opened up

after leaving the private mode by clicking again onto it the windows in the 'normal' mode come back and are opened up.
but the sortation on the taskbar is now reverse.

Example:
- starting firefox, open windows: A, B, C
- changing into private mode, open window: Z
- leaving private mode, opens up again: C, B, A

Xubuntu 11.10
Linux ubuntu 3.0.0-17-generic #30-Ubuntu SMP Thu Mar 8 20:45:39 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux
Firefox 12.0+build1-0ubuntu0.11.10.1

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: firefox 12.0+build1-0ubuntu0.11.10.1
ProcVersionSignature: Ubuntu 3.0.0-17.30-generic 3.0.22
Uname: Linux 3.0.0-17-generic x86_64
NonfreeKernelModules: fglrx
AddonCompatCheckDisabled: False
AlsaVersion: Advanced Linux Sound Architecture Driver Version 1.0.24.
ApportVersion: 1.23-0ubuntu4
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC1: frank 2389 F.... xfce4-volumed
 /dev/snd/controlC0: frank 2389 F.... xfce4-volumed
                      frank 2429 F.... pulseaudio
BuildID: 20120423122843
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 : 39
   Simple ctrls : 22
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: Sat Apr 28 15:28:48 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=2.3.9, 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:
 default via 192.168.0.1 dev eth0 proto static
 169.254.0.0/16 dev eth0 scope link metric 1000
 192.168.0.0/24 dev eth0 proto kernel scope link src 192.168.0.101 metric 1
IwConfig:
 lo no wireless extensions.

 eth0 no wireless extensions.

 vboxnet0 no wireless extensions.
ProcEnviron:
 PATH=(custom, no user)
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
Profiles: Profile0 (Default) - LastVersion=12.0/20120423122843 (Running)
RfKill:

RunningIncompatibleAddons: True
SourcePackage: firefox
UpgradeStatus: Upgraded to oneiric on 2012-03-26 (33 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.

[Launchpad Janitor (janitor)]

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

[LAZA (laza74)]

Bug is still active in Firefox 21.

After a crash of Fx, the windows are sorted upside down...