For over a year, YouTube pulled support for their app on FireTV, because Amazon wouldn't allow sales of Google Home devices on their site. So Google pulled YouTube support. One had to use the Amazon Silk browser to get to YouTube on their FireTV.

However, they seem to have resolved their issues. The first tile on my FireTV today was for YouTube app being available now. Sure enough, it is. It even has a silent preview when you navigate to a thumbnail. If it's music, there is also a sound preview. You can turn off that feature in the Settings.