I don't know how to do that, but the plugin source code is available in the .xpi file at
http://kinook.com/Download/URFFExtension.xpi (the .xpi file is a .zip file, and a .jar file inside of it has the extension code -- see
http://www.google.com/search?q=firef...on+development for more details). I believe others have used Scrapbook to import into UR (search the forum).
An imported web page and related files is stored in a proprietary format in UR and isn't easily accessible via an API.