Strange temporary display issue in Firefox

I was on a website scrolling up and down and I noticed a line of dots going up and down about 2/3 the way across the page from the left side. The more I scrolled up and down the more dots would appear until it was almost a line. I browsed to different webpages on other websites and it the problem continued. I opened these pages both in new tabs and the original tab. There seemed to be no rhyme or reason to it. The original tab had a comma in direct line with the line of dots, but they were not comma's just dots. I tried getting a screenshot, but if I touched the keyboard or clicked on anything other than the Firefox window, the dots would disappear. Scrolling the window up and down and a new line of dots would appear. At first I thought it was a graphics card issue, so I opened up a chrome window to see if it was, and there was no issue in chrome. So I went back to Firefox and it was still there. After 20 minutes of trying to track it down unsuccessfully, I closed Firefox and then opened a new instance of Firefox and the problem was gone. I have been unable to duplicate it since then.