like Arkiven4 said in some case is related to hardware acc setting.
you can try disable hardware acc.Firefox’s hardware acceleration feature uses your computer’s graphics card to render text and objects on web pages. this speeds up web page rendering and takes some load off your CPU. however, hardware acceleration can cause problems with some graphics drivers and graphics cards.
you can determine whether hardware acceleration is the problem by disabling it. To do so, click the Firefox button and select Options. Click the Advanced icon and uncheck the Use hardware acceleration when available checkbox.
note that you should not disable hardware acc if everything is working properly, as this is a useful feature.
