Greasy Fork is available in English.
Tells YouTube that your browser only supports videos at 30FPS or less, which switches all 60FPS videos to 30FPS and allows old computers to watch high-resolution videos without stutter!
< Feedback on Disable YouTube 60 FPS (Force 30 FPS)
This script did awesome job for me for years, but now (10.2020) Youtube changed something and now it's impossible to force switch 1080p60fps to 1080p30fps videos. 720p still seem to work though. I just hope it's still possible to fix this somehow. Anyway thanks Steve, without this script I wouldn't be able to watch HD YT videos for the past 3 years.