Favicons are the small shortcut icons on the Firefox browser. They are on the browser’s tabs and menus. You can remove favicons from Firefox by configuring a couple of options on the advanced settings page.
Open the Firefox browser, and enter about:config in the address bar. Press Return to open the page in the shot below. There you should click I’ll be careful, I promise!
Next enter browser.chrome in the search box at the top of the page. That opens the options in the shot below. There are a couple of settings there which when set to false switch the favicons off.
Firstly, right-click browser.chrome.favicons and select Toggle to switch its value to false. Also right-click on the browser.chrome.site_icons switch its value to false much the same. Next close and reopen the Firefox browser.
Once you’ve restarted Firefox, you will find that the Favicons are gone entirely. For example, note that there are no favicons on Firefox’s tabs in the shot below.
To restore the favicons, return to Firefox’s advanced setting page; and switch the browser.chrome.favicons and browser.chrome.site_icons values to true. You can also remove Firefox bookmark favicons with the Bookmark Favicon Changer  add-on.