I don't normally write reviews for things like this, because no one reads reviews for things like this. However I am compelled. This add-on has a few neat bells and whistles, but the bottom of the line, right off the bat, is this: you will use about 3 times as much memory (EDIT: After checking to confirm, most Youtube tabs with an open video without the add-on use about 35-45 MBs of memory, with the add-on video tabs were around 160-190 MBs of memory) for any Firefox tab you have opened to Youtube with this installed. You will run out of memory faster if you're system is lacking, and you will easily find your browser locking up with an open Youtube tab running in the background because of the insane amount of memory being used to run this add-on. Definitely NOT worth the bells and whistles. I essentially wanted an app to stop autoplay from ever activating, and this is the one thing the add-on seems to fail at, so it did not meet my needs in the first place, but once I saw how out of control it made Youtube, I was out, and I think anyone considering this add-on should really think about if it's worth it.
EDIT: After reading some other reviews I question whether or not Mozilla does anything to prevent fake reviews, and I especially question the legitimacy of a great deal of the reviews on this add-on.