YouTube Center is good software. It’s an unofficial browser extension to make YouTube work better. Works in most browsers; for Chrome you download the Opera .crx file and drag it into the Tools/Extensions page.
What does it fix? #1 thing is it lets you disable DASH playback, the nonsense YouTube implemented a couple of years ago. In theory DASH makes videos play faster and more efficiently; in practice it’s the crap that makes it impossible to pre-buffer a video or seek backwards while playing. YouTube Center also does a good job at resizing the video window to use more of the screen, so that a 720p video actually has a 720 row high window to play in. I also use it to prevent auto-play and to select the video resolution I prefer.
The main drawback is that there are too many configuration options, many of which you don’t need. Classic hackerware; the author lets you configure everything, so it’s up to the user to tune the few things they really need to set.
I’ve used a few “fix YouTube” extensions in the past that were flaky or broke when YouTube changed something. This one seems to be working for me. I don’t understand why Google’s let their video product get so crummy that it’s necessary to hack it like this.