After re-installing my PC I also installed a fresh Firefox and added all my standard Plug-Ins. Everything worked perfectly fine with perfect stability. Then all of a sudden, probably after clearing History all these plug-ins disappeared from the toolbar and also don´t work (seeing commercials on youtube etc.). Only 1 Plug-In is still displayed and still works (NordVPN). I´ve been looking for solutions but didn´t find anything on this problem. There´s one fact that makes it a little special: all of the Add-Ons are in the Menu/Add-On Tab and activated, no problems appear. (Screenshot)