Your screenshot is displaying glitchy, repeating patterns of video artifacts which are characteristic of video RAM artifacts.
If your computer screen is displaying video RAM artifacts, then it would show up first in applications that use a lot of RAM, like Firefox. If the RAM in your graphics card is failing, sometimes just a small fraction of the RAM goes bad while the rest of the RAM continues to work properly.
As a workaround in Firefox, click the Firefox Main Menu button ≡ in the top-right corner of the screen. Go to Preferences -> Advanced -> General tab, and under the Browsing heading uncheck the option Use hardware acceleration when available.
Update
The update to Firefox 54 enabled multiprocess support for content, including a GPU acceleration process. If your GPU has hardware defects, the separate GPU acceleration process can cause crashing as well as visual artifacts on some websites. On the other hand, this provides an additional source of information that helps to more easily diagnose possible GPU hardware defects.
2Can't repro on firefox 43.0.3/64 bit. Screenshots would be handy – Journeyman Geek – 2016-01-06T04:28:47.127
1what video card brand/model and driver version? – Richie Frame – 2016-01-06T12:39:04.900
5I thought this was me! finally a solution. – Martijn – 2016-01-06T13:16:06.830
1@RichieFrame I'm using ATI graphic card (1GB) – undone – 2016-01-06T18:34:25.337
1I had a similar problem with Internet Explorer 11 and the same OS, just it was horizontal lines. +1 for asking. – wizzwizz4 – 2016-01-06T18:47:05.587