The icon from the bookmark is determined by the website's favicon, which is set by the website developer. If the website developer has deleted the favicon or made a mistake in the code that prevents Firefox from properly reading/loading the favicon, then Firefox's default dotted line box will be displayed as the bookmark icon. Please navigate to the website. If there is no favicon displayed on the tab bar, then please contact the website developer about the issue. If there is a favicon in the tab bar, please try deleting the bookmark and bookmarking it again.