I was sure that the new firefox 40 was the problem. To make it worse in chrome the videos were not lagging at all, but when I restarted my computer it failed to restart properly. I saw this kind of problems with ram sticks before so I said to give it a try. The sticks were very hot and one of them showed signs of oxidation. I've cleaned them with a eraser and now the computer works great again.

I have 4gb ddr2 800 adata vitesta in 4x1gb sticks.