How to full disable GPU-acceleration (only) for videos (ex. on youtube) in browser?

My video card (HD4670 on Win7 x64, with Firefox stable x86), does not cope with the hardware processing of video 1080p 60fps, and starts to slow down and freeze. In other browsers, such as Opera, Chrome video works smoothly without brakes (apparently there it automatically turns off the GPU acceleration and the video is processed by the processor). In such a player, such as PotPlayer, for example, 1080p@60fps go smoothly, without jerking and loss of frames (with disabled DXVA and any GPU-acceleration). Help me solve this question, it already makes me angry. I've tried a lot of ways, but nothing helps to disable video acceleration for videos in Firefox.