A new plug-in has been released that installs quickly and adds an option to search Drupal from Firefox's built in search field (to the right of the URL field). Very useful if you're a Drupal user or developer.
Unfortunately, the site owner appears to have taken the page down which linked to these files. However, the files can still be downloaded at these links:
Drop the two files into your Firefox\searchplugins directory, restart Firefox, and you're all set to go.
You can also download the files from AnalogDuck.com.