Did you check the overflow area under the ">>" at the far right in case the icon is hidden there?

If you use Adblock Plus then you can consider uBlock Origin as a replacement to see if that works better (lower memory footprint).

• https://addons.mozilla.org/firefox/addon/ublock-origin/

In addition to the above :

Your system details show that you have AdBlock Plus installed.

Another thing to try :

Go to the 3-bar menu => Options => Customize

If the AdBlock Plus icon is there, you can drag and drop it to wherever you want it .....