YouTube SpeedUp od GoliafRS

YouTube SpeedUp adds a speedometer button to the YouTube player, on the right
side of the controls at the bottom. Scroll the mouse wheel over it to speed up
or slow down, click it to return to normal speed. Settings are on the options
page.

• Open the options page from the add-on's entry in Add-ons and themes
  (about:addons).
• Each wheel step changes the speed by 0.25. Choose another step on the options
  page.
• Point at the button to see the current speed.
• Speed from 0.25× to 16×.
• Shorts are off by default. Turn them on in the options page, and the button
  appears among the Shorts player buttons.
• Keep one speed for all videos, or a separate speed for each video. The options
  page lists the saved speeds, where you can change or remove them.
• Scrolling over the button never scrolls the page, even in fullscreen.
• Settings stay in your browser and are never sent anywhere.