How to Create a Firefox Search Plugin (Extension)

The Firefox browser (as of version 3.0.2 and likely earlier versions), there is a search bar at the top right. Custom search engine plugins are available to allow easier searching. For this example, I will demonstrate how to create search engine for the search engine Scour. We will begin with a template which should be copy and pasted into an xml file (in our case, it will be scour.xml).