Firefox also had an unrelated tearing issue for myself (my system has hybrid graphics intel/nvidia) but you may want to give this a quick try. Just go type about:config in the URL navigation bar, hit enter, accept the risks and search for and set gfx.xrender.enabled to TRUE (the default is False so if it gives you any problems, just change it back to that). You may have to restart Firefox to see if the changes fix anything.
Try that one option above first, and if maybe the issue still persists I've seen this work around for others, but it did not work for myself. The above fix was all my system needed to work just fine. So, the second backup option is to search for and set layers.acceleration.force-enabled to TRUE. If any issues however, set to False which is the default for this option. Hope perhaps this works for ya! Cheers!\
Edit: some words, my fingers never type what I'm thinking :-P
