From MozillaZine Knowledge Base
Internet Explorer 5 introduced favicons, a way to associate a 16x16 pixel icon with a given page or site. Mozilla gained support for these icons with BMP/ICO support in Mozilla 0.9.6. When enabled, favicons are displayed in the address bar, tabs, and in the bookmarks menu.
Mozilla considers favicons a type of “site icon.” Another type of site icon is the small image thumbnail displayed in the address bar and as the tab icon when directly viewing an image. (Those are not affected by this preference.)
Possible values and their effects
Display favicons in the address bar, bookmarks menu, and in tabs. (Default)
Display the default Mozilla “document” icon instead.
First checked in
Has an effect in