how do I get this version back?

Hello, I hope so much u guys can help.i use to b able to download youtube videos on firefox. I had a small grey arrow next to the plus, share and flag buttons just under the video on the description panel. I had to make some space and I think I either deleted firefox or deleted the data in the app manager. When u clicked on the arrow a box with the options appeared. E.g mp4 720, mp4 360 and I think at the bottom flac 220 or something like that. It worked every time and I loved it. Can any one help direct me to get it back. I cant recall using this addon page but I do remember downloading something I think. I'm lost with out it. Thank u so much :)