That can happen if the site doesn't have a favicon.
In older versions you used to get a folder icon in such a case, but now you see an square.

See:

• [/questions/890741#answer-267094]

Add code to userChrome.css in the chrome folder below the @namespace line.

• http://kb.mozillazine.org/userChrome.css
• http://kb.mozillazine.org/Editing_configuration

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#page-proxy-favicon:not(src),
.tab-icon-image:not(src) {
 list-style-image: url("chrome://global/skin/icons/folder-item.png")!important;
 -moz-image-region: rect(0px, 16px, 16px, 0px)!important;
}

But these sites had icon in FF7. For example, Facebook. It had a favicon, but in FF8 there is only a square.

Try the CheckPlaces extension.

• CheckPlaces: https://addons.mozilla.org/firefox/addon/checkplaces/

A possible cause is a problem with the file places.sqlite that stores the bookmarks and the history.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox
• https://support.mozilla.com/kb/Bookmarks+not+saved#w_places-database-file

It doesn't matter. Favicons won't show in the address bar or on the tab weather the site has a favicon or not. I ran CheckPlaces and it's still doing it. Maybe I'll try deleting places.sqlite to see if that works.

Interestingly, they still show in the bookmarks menu and history.

I just tried deleting it. Same problem.

Ok. I fixed it. I had to disable the Favicon Picker extension because it was interfering with it. It works fine now.