FaviconizeTabs is an excellent addon for those who keep multiple tabs open in their tab bar.
Sadly, after upgrading to Firefox 52.0, most of the tabs that are in the auto faviconize list in the auto faviconize list., are not iconized.
However, when a tab that is in the auto faviconize list and is not iconized is clicked, that tab iconizes.
Note that in Firefox 52, you cannot move the tabs around in the tab bar.
Firefox is becoming less and less, the browser for users and user convenience and more and more a browser whose developers dictate what users need based on their heightened perception of self importance that is so dominant in narcissistic communities.
Needless to say, the days when Firefox was the best browser ended when they took away the status bar, defaulted the hiding of the address bar, and forced upon users a tabs above address bar design.