Open Mozilla Firefox on your computer.
Go to "Tools" on the Menu bar and then select "Add-ons" from the drop-down menu to open the "Add-ons" window.
Select "Extensions," scroll down to "Better YouTube" and then select it.
Click the "Options" button, click the checked box to the left of "YouTube Alternate Video Player (No Autoplay)" in the box that appears and then the "OK" button to disable the player, but leave the rest of the add-on intact.
Click the "Disable" button under "Better YouTube" if you want to disable the entire add-on and the Flowplayer that it uses instead.
If "Restart Firefox to complete your changes" appears on a yellow bar above the add-ons list, click the "Restart Firefox" button so that the new change takes effect.