I prefer to watch videos in my favorite video player VLC instead of watching them embedded on the web. If you click on a video file on a website you open it either in your default video viewer or view it right in your browser using a plugin. You may also open websites in different browsers which is great for developers who were using several add-ons to view a website in different browsers.
Just right-click the link and select the application that you want to use to view it from the menu. Launchy, that is the name of the Firefox add-on, supports more than 60 applications right out of the box with the ability to add more applications using an XML file.
It is possible to disable programs that have been found and are selectable in the right-click menu.
If you for instance want to view the source you have the option to use Ultraedit, Notepad, Open Office or Wordpad which is pure overkill. It is convenient to remove all but one program unless you really use more than one source viewer.
I personally can see two great applications for Launchy. The first is for webmasters who like to test websites in several browsers. The second is for users who would like to open media files in external applications and for users who like to send images right into their image editing software.
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For media files you may use MediaPlayerConnectivity add-on. It is context-sensitive.