Yes, I liked 8.1, but last year Windows Store stopped working and because I couldn't fix it, I decided to try Windows 10 for one more time, even though my previous attempts had ended with pretty quick uninstalling. I have to say there are still several problems with Windows 10, but this time I decided to stick with it just because otherwise I would feel like being a dinosaur soon.
Regarding my video issue, it's solved. The problem was in my computer video card. Firefox just doesn't use as much resources as Opera and Chrome while streaming videos. That's why everything worked with Firefox, even though the video card was defective.
Thank you for everyone who read about the issue.