1.Open any of the desired browser and then download and install SmartVideo For YouTube extension from web store.
2.After installing the extension, a Welcome tab from SmartVideo for YouTube will get opened.Now scroll down to the bottom, you would see an option called Set global preferences now underPreferences.Tap on that and make sure whether the smart buffer option is checked if not just check the smart buffer boxes.
3.If welcome tab does not appear you can edit global preferences while playing YouTube video.Just head over to YouTube and play the video which you wish to watch.
Move the cursor over the video, other than normal video related options you could see some extra options popping out under the video.You can even set the smart buffer option here if you have not done step 2.
Other than these changes you need not to modify anything, the smart buffer option will buffer the video quickly than the normal buffering speed before data reaches the buffering point, thus allowing you to watch any YouTube videos continuously without buffering.So by using this simple extension method you can easily watch any videos hassle free.