Firefox slow to display contents of bookmarks folder

My 5 year old 2MB Dell Dimension E520 desktop used to be quick for Lucid's Firefox to display contents of a bookmarks folder when that folder's name was clicked on (under the Bookmarks menu entry). Since doing a clean install (with all packages updated) of Precise last week, it's slow (can take 10+ seconds perhaps depending on number of entries in a folder) to display contents of bookmarks folder. When I disable the Unity Global Menu Bar for Firefox & Thunderbird, behaviour is back to normal. This applies to latest version of Menubar (3.6.4 installed just now) as well as previous version.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: firefox-globalmenu 18.0+build1-0ubuntu0.12.04.3
ProcVersionSignature: Ubuntu 3.2.0-35.55-generic-pae 3.2.34
Uname: Linux 3.2.0-35-generic-pae i686
AddonCompatCheckDisabled: False
AlsaVersion: Advanced Linux Sound Architecture Driver Version 1.0.24.
AplayDevices:
 **** List of PLAYBACK Hardware Devices ****
 card 0: Intel [HDA Intel], device 0: STAC92xx Analog [STAC92xx Analog]
   Subdevices: 1/1
   Subdevice #0: subdevice #0
ApportVersion: 2.0.1-0ubuntu15.1
Architecture: i386
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: john 1836 F.... pulseaudio
BuildID: 20130107224926
CRDA: Error: command ['iw', 'reg', 'get'] failed with exit code 1: nl80211 not found.
Card0.Amixer.info:
 Card hw:0 'Intel'/'HDA Intel at 0xdffdc000 irq 44'
   Mixer name : 'SigmaTel STAC9227'
   Components : 'HDA:83847618,102801dd,00100201'
   Controls : 38
   Simple ctrls : 21
Channel: Unavailable
Date: Wed Jan 9 10:03:40 2013
ForcedLayersAccel: False
IfupdownConfig:
 auto lo
 iface lo inet loopback
InstallationMedia: Ubuntu 12.04.1 LTS "Precise Pangolin" - Release i386 (20120817.3)
IpRoute:
 default via 192.168.1.1 dev eth0 proto static
 169.254.0.0/16 dev eth0 scope link metric 1000
 192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.2 metric 1
IwConfig:
 lo no wireless extensions.

 eth0 no wireless extensions.
MarkForUpload: True
PrefSources:
 prefs.js
 [Profile]/extensions/{b9db16a4-6edc-47ec-a1f4-b86292ed211d}/defaults/preferences/prefs-dwhelper.js
ProcEnviron:
 LANGUAGE=en_GB:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
Profiles: Profile0 (Default) - LastVersion=18.0/20130107224926 (In use)
RelatedPackageVersions:
 rhythmbox-mozilla 2.96-0ubuntu4.2
 totem-mozilla 3.0.1-0ubuntu21.1
RfKill:

RunningIncompatibleAddons: False
SourcePackage: firefox
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 03/30/2007
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 2.3.2
dmi.board.name: 0WG864
dmi.board.vendor: Dell Inc.
dmi.chassis.type: 6
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvr2.3.2:bd03/30/2007:svnDellInc.:pnDellDM061:pvr:rvnDellInc.:rn0WG864:rvr:cvnDellInc.:ct6:cvr:
dmi.product.name: Dell DM061
dmi.sys.vendor: Dell Inc.