System memory thrashes when Firefox accesses certain websites

[Amapele]

See Question #198533 for details on problem reproduction and some memory statistics. Recent Ubuntu updates have made the situation slightly better:
* the Firefox window only occasionally grays out if at all and
* mouse control is retained although movement is still jerky about 75% of the time.
Now I can end the Firefox process (through System Monitor) without rebooting most of the time.

Testing has shown that running Firefox without extensions, plugins, and JavaScript and in safe mode does not bypass the problem. Running with the RAMBack addon or a new profile also does not help. The about:memory page cannot be accessed after thrashing starts. System Monitor shows Firefox using less than 250MiB and the ten next largest memory users using only about 30MiB total on a 512MiB system.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: firefox 12.0+build1-0ubuntu0.12.04.1
ProcVersionSignature: Ubuntu 3.2.0-24.39-generic-pae 3.2.16
Uname: Linux 3.2.0-24-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: ALC880 Analog [ALC880 Analog]
   Subdevices: 1/1
   Subdevice #0: subdevice #0
ApportVersion: 2.0.1-0ubuntu8
Architecture: i386
ArecordDevices:
 **** List of CAPTURE Hardware Devices ****
 card 0: Intel [HDA Intel], device 0: ALC880 Analog [ALC880 Analog]
   Subdevices: 1/1
   Subdevice #0: subdevice #0
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: charlie 1814 F.... pulseaudio
BuildID: 20120423122624
CRDA: Error: command ['iw', 'reg', 'get'] failed with exit code 1: nl80211 not found.
Card0.Amixer.info:
 Card hw:0 'Intel'/'HDA Intel at 0xdfffc000 irq 46'
   Mixer name : 'Realtek ALC880'
   Components : 'HDA:10ec0880,00000000,00070400'
   Controls : 9
   Simple ctrls : 5
Channel: release
Date: Tue Jun 5 11:10:54 2012
ForcedLayersAccel: False
IfupdownConfig:
 auto lo
 iface lo inet loopback
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release i386 (20120423)
IpRoute:
 default via 10.0.0.1 dev eth0 proto static
 10.0.0.0/24 dev eth0 proto kernel scope link src 10.0.0.4 metric 1
 169.254.0.0/16 dev eth0 scope link metric 1000
IwConfig:
 lo no wireless extensions.

 eth0 no wireless extensions.
ProcEnviron:
 PATH=(custom, user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
Profiles: Profile0 (Default) - LastVersion=12.0/20120423122624 (Running)
RfKill:

RunningIncompatibleAddons: False
SourcePackage: firefox
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 07/22/2004
dmi.bios.vendor: Intel Corp.
dmi.bios.version: EV91510A.15A.0067.2004.0722.1717
dmi.board.name: D915GAG
dmi.board.vendor: Intel Corporation
dmi.board.version: AAC77881-302
dmi.chassis.type: 3
dmi.chassis.vendor: Gateway
dmi.modalias: dmi:bvnIntelCorp.:bvrEV91510A.15A.0067.2004.0722.1717:bd07/22/2004:svnGateway:pn:pvr:rvnIntelCorporation:rnD915GAG:rvrAAC77881-302:cvnGateway:ct3:cvr:
dmi.sys.vendor: Gateway

[Launchpad Janitor (janitor)]

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