Firefox 10 shows bad css when the sidebar is open

I usually open Google Translate in the sidebar
http://translate.google.cl/

I realized that when I do that, the actual page change the way it show the elements in the page.
For example, look at
http://www.ruby-doc.org/

Notice that when the sidebar is open, the top-right gray bar is not well-displayed.
This also happens in the page loaded in the sidebar.

I'm using Firefox 10.0.2, in ubuntu 11.10 x64.

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: firefox 10.0.2+build1-0ubuntu0.11.10.1
ProcVersionSignature: Ubuntu 3.0.0-16.29-generic 3.0.20
Uname: Linux 3.0.0-16-generic x86_64
AddonCompatCheckDisabled: False
AlsaVersion: Advanced Linux Sound Architecture Driver Version 1.0.24.
ApportVersion: 1.23-0ubuntu4
Architecture: amd64
ArecordDevices:
 **** List of CAPTURE Hardware Devices ****
 card 0: Intel [HDA Intel], device 0: ALC272X Analog [ALC272X Analog]
   Subdevices: 1/1
   Subdevice #0: subdevice #0
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: carlos 13063 F.... pulseaudio
BuildID: 20120216115113
Card0.Amixer.info:
 Card hw:0 'Intel'/'HDA Intel at 0xf0800000 irq 42'
   Mixer name : 'Intel IbexPeak HDMI'
   Components : 'HDA:10ec0272,10250370,00100001 HDA:80862804,80860101,00100000'
   Controls : 24
   Simple ctrls : 13
Channel: release
Date: Tue May 1 20:02:11 2012
ForcedLayersAccel: False
IfupdownConfig:
 auto lo
 iface lo inet loopback
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)
IpRoute:
 default via 192.168.0.1 dev wlan0 proto static
 169.254.0.0/16 dev wlan0 scope link metric 1000
 192.168.0.0/24 dev wlan0 proto kernel scope link src 192.168.0.103 metric 2
ProcEnviron:
 PATH=(custom, no user)
 LANG=es_CL.UTF-8
 SHELL=/bin/bash
Profiles: Profile0 (Default) - LastVersion=10.0.2/20120216115113 (Running)
RunningIncompatibleAddons: False
SourcePackage: firefox
UpgradeStatus: Upgraded to oneiric on 2011-12-15 (138 days ago)
dmi.bios.date: 05/04/2010
dmi.bios.vendor: Phoenix Technologies LTD
dmi.bios.version: V1.12
dmi.board.asset.tag: No Asset Tag
dmi.board.name: Aspire 4741
dmi.board.vendor: Acer
dmi.board.version: V1.12
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 10
dmi.chassis.vendor: Acer
dmi.chassis.version: V1.12
dmi.modalias: dmi:bvnPhoenixTechnologiesLTD:bvrV1.12:bd05/04/2010:svnAcer:pnAspire4741:pvrV1.12:rvnAcer:rnAspire4741:rvrV1.12:cvnAcer:ct10:cvrV1.12:
dmi.product.name: Aspire 4741
dmi.product.version: V1.12
dmi.sys.vendor: Acer