The One; The Original Fasterfox Gives performance & network tweaks for Firefox. Fasterfox makes your browsing so much faster!
Make your Sites Faster than a Fox with Fasterfox for FireFox!
https://addons.mozilla.org/en-US/firefox/addon/rsccmanfasterfox/
YouTube Center contains all kind of different useful functions which makes your visit on YouTube much more entertaining.
In this addon go to settings and uncheck DASH playback in player setting and external player setting
https://addons.mozilla.org/en-us/firefox/addon/youtube-center/