It happens vertically & horizontally; in passive viewing & active use (such as while typing). Text & pictures devolve into long lines (think paint dripping down a wall in streams). I took a few screen shots to illustrate. Just started doing this after an update 2 days ago. After trying unsuccessfully trying to solve the problem I removed Firefox from my computer & reinstalled it, but the same thing continues to happen. I tried opening the same sites & email messages & doing all the same activities in another browser to double check & found none of the same problems. I tried out many programs on my own computer to determine if it was a problem specific to my computer & found none of those problems (in Word, in Adobe, using various image & text programs). It seems to be specific to Mozilla Firefox -- & seems to have to do with an upgrade of some kind on Sept 22nd, 2016.