This behavior has just started recently. I suspect Avast may be involved because of the colors involved. When I do something that opens a popup window, a black bar appears at the top of the popup window asking me if I want to block it. There's a submenu which allows me to block or allow a domain, a subdomain, or a URL. I tried disabling web shield, but I still see the black bar. This is on Firefox 58.0.2 (64-bit) on OSX High Sierra. I looked through the preferences in Avast Mac Security, but I don't see any that would turn this behavior off.
