So far, Facebook, Youtube and Vine aren't affected. Yesterday, Firefox stopped playing GIFs (which are actually displayed as videos) on Twitter. I can see the play button and the "GIF" word on the corner, but then the image turns black and the following message appears: "This browser does not support video playback.". I honestly have no clue how it started. One day it was working fine as usual, but then it happened. Refresh feature didn't solve the problem, and entering Safe Mode didn't work either. It seems it is a rare problem, since I can't find any solutions on the Internet and I tried asking if this was a known issue to Firefox account on Twitter, but I had no answer. This is seriously annoying, but I noticed another browser, the Vivaldi, also has the same problem, but Google Chrome doesn't. This is really confusing. Can someone help me?