To get a favicon to show up after you have bookmarked a site, you must click on the bookmark to go to the site. Using the bookmark will cause it to check for a favicon.
After Google changed their favicon, I received the updated icon after I used the bookmark to access the site.
If you open the bookmarks.html file stored in your profile area, you will see each bookmark that has an icon has a section labeled: ICON="data:image/x-icon;base64,
The icon image is stored with the bookmark in a base64 format. You can cut the data that follows the comma into notepad, save the file as "test.b64" and open it with WinZip. This will show a file named "Unknown.001". Save this file to your desktop and rename it something like "icon.ico" Windows should show you the icon bitmap as the thumbnail of the file. If you have a program that will allow you to edit .ico files, you can open the file and verify it is the same 16x16 image Firefox uses for the favicon.