Firefox users who frequent YouTube, may be interested in the extension: Better YouTube.

[This extension] enhances YouTube with Greasemonkey user scripts, that offer options like the FlowPlayer “doesn’t start till you hit play” viewer, clean “theater” view, and links to download the video automatically.

With this add-on installed, you get features such as:

• Download link on the page.
• Ability to prevent auto-play (start by clicking play).
• Optimized screen which enlarges video and removes other page elements.
• Hide comments and more videos section.

If any of these features sound useful to you, give it a try.