firefox and xorg high cpu usage when render html5

What wrong:
When open in firefox webpage with html5 video
(any video on youtube.com or site like http://belzen.by/ http://zenrus.ru/)
you get high cpu usage by xorg and firefox

Tasks: 230 total, 2 running, 228 sleeping, 0 stopped, 0 zombie
%Cpu(s): 23,7 us, 6,9 sy, 0,0 ni, 69,3 id, 0,1 wa, 0,0 hi, 0,0 si, 0,0 st
KiB Mem: 5965988 total, 5640908 used, 325080 free, 606536 buffers
KiB Swap: 6148092 total, 44676 used, 6103416 free. 2335352 cached Mem

  PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
16218 root 20 0 510160 155508 82312 R 62,4 2,6 82:31.02 Xorg
17749 zed 20 0 2866288 621124 121032 S 36,2 10,4 149:36.32 firefox
17635 zed 20 0 1305816 384248 84996 S 8,0 6,4 135:20.26 thunderbird
16794 zed 9 -11 521140 11508 8900 S 4,6 0,2 0:41.63 pulseaudio
16867 zed 20 0 1675388 258500 86848 S 4,0 4,3 133:39.48 compiz

bug exist on 2 different PC (14.04.2 and 14.04.3) with mesa or proprietary amd driver .
if use Chromium browser - all work perfect.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: firefox 40.0+build4-0ubuntu0.14.04.4
ProcVersionSignature: Ubuntu 3.19.0-26.28~14.04.1-generic 3.19.8-ckt4
Uname: Linux 3.19.0-26-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.14.1-0ubuntu3.12
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/pcmC0D0p: zed 16794 F...m pulseaudio
 /dev/snd/controlC0: zed 16794 F.... pulseaudio
 /dev/snd/controlC1: zed 16794 F.... pulseaudio
BuildID: 20150819100600
Channel: Unavailable
CurrentDesktop: Unity
Date: Mon Aug 24 09:37:06 2015
ExecutablePath: /usr/lib/firefox/firefox
ForcedLayersAccel: False
IfupdownConfig:
 # interfaces(5) file used by ifup(8) and ifdown(8)
 auto lo
 iface lo inet loopback
IncompatibleExtensions:
 English (South Africa) Language Pack - <email address hidden>
 English (GB) Language Pack - <email address hidden>
 Russian (RU) Language Pack - <email address hidden>
 Default - {972ce4c6-7e08-4474-a285-3208198ce6fd}
InstallationDate: Installed on 2014-12-02 (264 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
IpRoute:
 default via 10.0.14.104 dev eth0 proto static
 10.0.12.0/22 dev eth0 proto kernel scope link src 10.0.13.5 metric 1
 192.168.241.0/24 dev zt0 proto kernel scope link src 192.168.241.4
MostRecentCrashID: bp-5f8cd26b-298e-4d12-af4a-72aaf2150707
PrefSources: prefs.js
Profiles: Profile0 (Default) - LastVersion=40.0/20150819100600 (In use)
RelatedPackageVersions:
 totem-mozilla 3.10.1-1ubuntu4
 icedtea-7-plugin 1.5-1ubuntu1
 rhythmbox-mozilla 3.0.2-0ubuntu2
RunningIncompatibleAddons: True
SourcePackage: firefox
SubmittedCrashIDs: bp-5f8cd26b-298e-4d12-af4a-72aaf2150707
UpgradeStatus: No upgrade log present (probably fresh install)
WifiSyslog:

dmi.bios.date: 07/09/2014
dmi.bios.vendor: Hewlett-Packard
dmi.bios.version: 68CDD Ver. F.50
dmi.board.name: 1726
dmi.board.vendor: Hewlett-Packard
dmi.board.version: KBC Version 29.38
dmi.chassis.type: 10
dmi.chassis.vendor: Hewlett-Packard
dmi.modalias: dmi:bvnHewlett-Packard:bvr68CDDVer.F.50:bd07/09/2014:svnHewlett-Packard:pnHPProBook6540b:pvr:rvnHewlett-Packard:rn1726:rvrKBCVersion29.38:cvnHewlett-Packard:ct10:cvr:
dmi.product.name: HP ProBook 6540b
dmi.sys.vendor: Hewlett-Packard