It basically combines several Greasemonkey scripts together into a single package.
- smartly enlarges videos for better viewing
- hides user comments
- declutters the page
- disables autoplay
Download and install the addon, restart Firefox and you’re set to go. To edit the preferences you need to go to:
Tools -> Add-ons
And choose ‘preferences’ for the “Better YouTube” extension.
Some nice options there, I added a screenshot below of the “Theater View”