When I activate the the Avast Online Security plugin in Firefox a whole new bar opens up with one lonely icon. You can move the icon to another bar but it doesn’t stay there and the next time you use FF it popup again. Not a fatal flaw but it does use up some screen real estate. Generally speaking, I leave it off. Is there anyway to fix this so this situation doesn’t occur? Thanks.
Disable it completely seems to fix it quite nicely. 
In the interest of accuracy (or pedantry) AOS is an Extension, not a PlugIn.
When I was using it, I never had any trouble placing it where I wantes it and having it stay there. I no longer use it.
It is an extension. My apologies. My solution will be to disable it. There appear to be no adverse consequences when disabling it or for that matter benefits by enabling it.