Recently, in my Optiplex 960 MT, my add-on graphics card (AMD Radeon HD 6570) started failing and I removed it and went back to using the on board graphics (Intel 4 Series Express Chipset, GMA 4500. Driver version 18.104.22.16820 from 7-23-2012).
I now have a problem that makes it very difficult to use...when I scroll down a page, certain sections will turn into long vertical lines that seem to emanate from the text above. So parts of the page will be filled with nothing but these long vertical lines, or alternatively just be blank. This is when using Firefox 52.9 ESR. If I switch to Sea Monkey 2.49.4, the problem disappears and the viewing goes back to normal.
The only thing I've tried is to change the hardware acceleration, but this made no difference.
Before I installed the Radeon HD 6570 graphics card, I was using the same on board Intel graphics...and didn't have this problem with the long vertical lines.
What operating system are you using on this 11 year old model? Check your Dell Driver page and use the Dell drivers for the on board graphics instead of using Intel's. Dell customizes the drivers for your motherboard.