If you’ve gone to Add-ons and clicked Plugins in the left pane, but don’t see Avast listed on the right, then…

  1. Close all browsers.

  2. Go to Control Panel → Programs and Features and select Avast. Click Uninstall/Change. Select Change in the window that comes up and click next. Uncheck the Browser Security feature, and click next.

  3. When that’s finished, change it again, checking the Browser Security feature to install it again.