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.