Tap
about:config into the address bar and hit enter. Search for the preference titled "browser.chrome.site_icons" (use the little filter field at the top to search for it). Right click the preference and click "Toggle" to change the value from "true" to "false"
Restart Firefox and the changes will take effect
Edit: If you end up feeling lonely without the icons, doing exactly the same again (clicking "Toggle" to change the value from "false" to "true") then restarting Firefox once more will get them all back as if nothing ever happened
*av