I noticed that when I play youtube videos through firefox there is often stop and go because the download was not quick enough. I thought it was my ISP that had the problem. I even opted for faster speed which helped some. But the problem was there more often than not. So I got curious and did a Youtube-firefox search and found other people were having this problem. And there was a fix, and this worked for me as well. Now my download of a video gets way ahead of the play since I applied it so I though it would be good to post this here.
"The quick fix for this problem, at least for my own sake, is to increase the time between each of the saves performed by session restore. By opening about:config in your Firefox address bar, then typing browser.sessionstore.interval in the filter box, you'll see a value of 10000, which is in milliseconds. (Meaning your session is saved every 10 seconds.) I changed this to 300000, or every 5 minutes, as I don't have the urgent need for tab restoration. If you feel like being more on the safe side, try increasing it to something a bit lower, say 120000, or every 2 minutes.
You right click on the setting and select modify, then change that number to the one listed above. I set mine to the higher number and it works great. No more pausing of my videos on youtube.