This userscript let you control autoplay and playback quality on YouTube.

The script uses the official YouTube Player API. That means, it works seamlessly and is probably more stable than other approaches.

Tested in Firefox 11.0, Opera 11.62, Chrome 18.0.

Manual

A new button with a "cogwheel" icon should appear below the video player. When you click on the button, a panel will show up with three options:

Quality

You can choose from 5 different options to set the playback quality at page load:

AUTO: Let the player choose the proper quality. (default)
LOW: Set the lowest possible quality.
MEDUM, HIGH: Set the quality to 480p, 720p max.
HIGHEST: Set the highest possible quality.

Size

You can set the default player size:

AUTO: Let YouTube handle the player size. (default)
WIDE: Set the wide size as the default.
FIT: Set the player width the same as the page width, keeping the aspect ratio.

Auto play

You can control when to start playing the video:

ON: The video starts playing as soon as possible. (default)
OFF: The video starts when you click on the play button, or anywhere inside the video.
AUTO: The video starts playing when the page containing the player gets focus the first time. This means, when you open a video in a new window, or in a new tab, the video won't start until the page become visible.

Translations

English, Hungarian, Dutch (by Mike-RaWare), Spanish (by yonane), German (by xemino), Portuguese (by Pitukinha), Greek (by TastyTeo), French (by eXa), Slovenian (by Paranoia.Com), Russian (by an1k3y)

Related

Userscripts.org
Gresemonkey Firefox addon

Source

Changelog