In Firefox, you can turn off the favions that show up in the address bar of the browser as well as in your bookmarks.

Turn to off favions, type about:config in the address bar and hit Enter.

Look for the following two entries: browser.chrome.favicons and browser.chrome.site_icons and if they are not set to false double click on them until they are set to false.

After that you need to exit Firefox and restart it. And when you do, the favicons will not longer be shown.