Looks like your profile icon for the new profile management feature that has rolled out. [[Manage Firefox profiles|This article]] explains how to customize your profiles including changing the icon used for them. I'm not sure if it's possible to remove the profile icon if you have more than one of them (the icons are supposed to help you identify which profile you're using). If you delete all profiles but one (also explained in the article) and relaunch the browser it should go away, but if you create multiple profiles it'll come back again.