Mozilla's WebRunner is a single-serving version of Firefox that strips away all the bells and whistles. There's no Web surfing to be done with this lightweight tool. Menus, extensions, themes, toolbars, and navigation have all been excised, like a sculptor cutting away excess marble.
What you're left with is a Site Specific Browser for Windows, Mac, or Linux that uses bookmark files with the WEBAPP extension. The installer configures these files to open in WebRunner, but there's no "launch program" icon or option. You just double-click on a WEBAPP file you've downloaded or created, and off you go, ready to get to work without getting distracted by the temptation to surf anywhere else.
Mozilla has provided bundles for eight Web sites, including most of the Google Web apps, Twitter, and Facebook. They've also posted user-created bundles for Yahoo Mail, Meebo, and Bloglines, among others.
The Options menu is the only conceit and lives behind a gear icon in the lower-right corner of the program frame. Clicking on it reveals some basic commands: Print, an About page, a DOM Inspector, the Error console, and Install to Desktop. This last command saves the page you're looking at as a shortcut on your desktop. So, assuming you can figure out a way to get to a site that doesn't have a bundle--it's not that hard--you can create a WEBAPP shortcut link for it.
It's an interesting idea to tear just about everything out but the essentials, but if you lack the willpower to stay away from YouTube in a regular browser, what's going to stop you from just loading the standard version of Firefox and getting distracted in the first place?