Not sure if this is what your problem is, but there are known issues under investigation, and turning off HardWare Acceleration, or upgrading your drivers is worth trying.

• See Firefox is slow after updating to Firefox 38

(I will tag this question with bug1166066 )

• Does the problem get better with Hardware Acceleration off ?

Sometimes if the developers discover an incompatibility with a particular driver version it can be designated as "blocked". Is there any indication of that when you review the support information page?

If necessary, you can try forcibly switching YouTube from the HTML5 player to the Flash player using an add-on such as one of the following:

• https://addons.mozilla.org/firefox/addon/youtube-flash-video-player/
• https://addons.mozilla.org/firefox/addon/youtube-flash-player/

Flash has its own hardware acceleration code.

John99 - thanks for the answer. Unfortunatelly, I did both before. No visible differences with or without "acceleration" checkbox ticked. The same as regards NVidia drivers. My question actually - how to force FF to USE acceleration (since in the "technical windows" it is reported (translated from Polish): "Windows with HW acceleration: 0/1 Basic (OMTC)". AFAIK GeForce should be supported...

jscher2000 - thanks as well. Flash movies act pretty the same as in case of HTML5. As to the "blocked" list - I assume you're talking about this: https://wiki.mozilla.org/Blocklisting/Blocked_Graphics_Drivers - no GF 7600 for Windows 7 there...

I don't know, maybe the mechanism for acceleration is completely different outside and inside of a browser and I should not judge FF performance by what I can see in a simple video file player (app. 50% of CPU used max, even for full HD movies).

I noticed a preference that sounds as though it might turn on HA when it isn't enabled by the regular setting. It might cause crashing if HA has been disabled for a good reason... in that case start up in Firefox's Safe Mode to revert the setting.

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste gfx and pause while the list is filtered

(3) Double-click the gfx.direct2d.force-enabled preference to switch it from false to true

That probably won't take effect during your current session, but after you exit and start Firefox up again.

For possible future reference in using Firefox's Safe Mode:

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

That should disable HA even if you force-enabled it.