So here's how I got this extension to work.

1. Click the extension on an open website
2. Select the particular favicon you want. The URL will automatically be copied
3. Open the properties for the particular bookmark you want to use.
4. Paste the favicon URL into the URL of the bookmark and hit save
5. Reload the webpage so that the favicon image is displayed on the screen
6. Now replace the URL of the bookmark to the ORIGINAL URL
7. Click the bookmark.