This is quite simply the very best,most reliable and the easiest to use add on for Youtube I've yet to come across.
I find it utterly indespensible and use it all the time.
Not only that if you pay the nominal price ask to for go the ads aswell as get full on 1080pHD downloads then it does everything you could want.

Now if that's not enough whenever those slippery lot at Youtube upgrade the site this add on is back in business working with the latest update within a day or so,

!00% fully recommend to all.

Now I'm sure there's any number of firefox add-ons that I've used with this browser over the years but not one of them has come close to being used as much as this one nor remotely as useful or straight forward.