HD Youtube videos lagging on Firefox

[Valle641]

So i've got a new laptop from school and HD youtube videos are lagging like HELL on Firefox and I can't for the life of me figure out the issue, if anyone has any other solutions to this problem please post them here, also happy to troubleshoot this issue.

(Keep in mind the PC is just 2 weeks old)
Here is what i've tried:
Google Chrome plays HD videos just fine
I have 2 GPUs in this PC, a Nvidia GT720M and Intel HD graphics 4600, Both behave the same.
Tried to increase the “browser.sessionstore.interval” option in About:config since it worked for some people, nothing.
Disabled Hardware acceleration in Firefox, no effect.
HTML5 already enabled
Disabled flash on youtube.com, nothing
Only happens on 720p and up, 480p work totaly fine(but who wants to watch in that crappy quality)

System specs:
OS:
Windows 7 Enterprise 64-bit
Gpu:
Nvidia GT720M
Intel HD graphics 4600
Hard drive:
SAMSUNG MZ7LF192HCGS-000 SCSI Disk Device (SSD)
CPU:
Intel(R) Core(TM) i5-4210M CPU @ 2.60GHz
RAM: 8GB DDR3 @ 1600MHz

 

[Tradesman1]

Check in settings to see if Harware accelleration is on or off and try the other, most have it on and are fine, but on some having it on can screw things up

 

[Valle641]

Check in settings to see if Harware accelleration is on or off and try the other, most have it on and are fine, but on some having it on can screw things up

What settings do you mean, I have already (as written above) disabled Hardware accelleration in firefox and it didn't help)

 

[0xBahaa]

May I suggest trying to play youtube videos using Flash instead of HTML5?
Here is how to:
1- Install the latest version of flash player from adobe through this link:
http://get.adobe.com/flashplayer
2- Install a firefox addon called "youtube flash video player" to force firefox to open youtube vids with flash instead of HTML player.
search for it here:
https://addons.mozilla.org/en-US/firefox/

Try it and tell me how it goes so that we could pinpoint the problem.