I have been having this problem since Monday after my computer was shut down two days in a row when I didn't have access to it. After I had started it back up, I went straight to YouTube. I clicked on a video and it started loading but then Firefox stops responding, and after about 30 seconds or so, it starts responding again. I just thought it did this because I had been gone for two days, so I decided to scroll down and look at the comments, where it did the same thing. I started to get frustrated, so I decided just to stay up where the video was and I tried to change the quality, where Firefox stopped responding and then started responding again. I then decided just to restart my computer, which didn't help at all. I then downloaded Chrome to see if it did the same thing when I was on YouTube, and it didn't. I tried looking up a problem on here, and found a guy with my exact problem, who figured out that he could fix it by just logging out and logging back it, which I tried, but it didn't work for me. I also cleared my cache and my YouTube watch and search history, which temporarily helped, but then it went back to not responding then responding again. My current version of Firefox is 40.1 if that helps any.