Everything that I mentioned before can be added or changed using small Greasemonkey scripts. All you need to do is to install the Greasemonkey script for Firefox and add the following scripts to it:

• Download Video – adds a small bar to the top that displays a list of all videos that can be downloaded from that page.
• Youtube Junk Remover – removes Advertisements, Director’s Videos and special offers.
• Youtube Maximize – Adds a ‘maximize’ link on top of YouTube videos, so that you don’t have to use the built-in maximize button. This way you can middle-click to open full-screen videos in new tabs instead of new windows, keep your toolbars, and have several videos maximized at the same time.
• Youtube Remove Comments – removes all comments.
• Youtube Display all Comments – if you like comments. This displays all comments on one page.
• Youtube stop video download – adds a stop button to Youtube that stops the download of a video. Previously you could only use Pause to stop the playback but the video would still be transferred in the background.

Update: The userscript is no longer available. Firefox users can install the Firefox add-on TubeStop to stop the auto-playing of YouTube videos.