I'll be generous in considering that the developer of this add-on has to be very aware of frequent updates to Youtube and HTML embedded videos and make updates to the add-on in tandem. What I cannot be generous about, however, is how the add-on works in normal operation. So consider this, perhaps, an update on the status of the add-on as of the date of this review listed above.
For one, the percentage buffered on the video is set to 42%, which I can agree is probably an average amount of required buffering of the video meant to guarantee a seamless viewing experience. The Smart Buffer feature, as it's described in the UI, presumably allows for the add-on itself to determine at what percentage buffering should stop. I can't tell whether the SmartBuffer intends to change the percentage and make it visible for the user, or if it simply fixes the percentage and forbids you to change it. It seems to work differently between the web page and pop-up that both control the preferences. For that matter, they don't seem to work together, or be aware of one another, which I would see as a design fault, rather than a bug. I use this issue not as a primary complaint, but as an example of how the add-on fails to account for corner cases in frustrating ways.
It doesn't seem to handle ads very well. I would recommend using Adblock while running this add-on on Youtube videos, because without Adblock running, I can't tell whether the add-on wants to try to skip ads on its own accord, or let the ad play, while failing to load it. Ads have been playing at the beginning of Youtube videos for a long while now; I cannot imagine how this add-on hasn't accounted for them to date, or, if it does, why it seems to take so long to handle them.
My biggest complaint may be a bug, but again, it's one that should have been fixed a while ago, and is the main reason I downloaded this add-on: Youtube will autobuffer to a certain percentage ahead of your place in your video when you pause, and this add-on is meant to prevent that from happening. I have a pretty unstable Internet connection where I normally watch Youtube videos, so this add-on would have worked wonders for me if I could pause the video, wait a bit for the full video to buffer, and then continue watching.
The add-on does not buffer the full video. At the very least, it doesn't do it consistently, because I have had videos that perform this function quite usefully, but other times, the video will buffer as standard, notify that it is done buffering at my 100% preference, and the 20 minute video I had waited to watch was only buffered by about a minute.
The fact that the preferences page can break the Firefox window, preventing so much as scrolling or tabbing, was the straw that broke this weary camel's back. Consider contacting the developer to let them know about these issues. I will be sure to, as well, because if these problems aren't fixed, I couldn't recommend this add-on to anyone, and I definitely can't recommend it at this point.