After reinstalling my system (Win 7 x64) and downloading the latest Nightly build of Firefox, logging in to Sync and syncing my bookmarks, I discovered that the icons are missing (white square with gray outline). After visiting a web page, the icon is set back to normal. The problem is, that I have lots of bookmarks, and it would take an enormous amount of time to visit every page. And on the second hand, with icons its easier to access bookmarks.