Thank you for making such a usefull addon.
Fast, easy to use and very customisable.

Problem:
Unfortunately because Firefox does not currently have a built-in way to get favicons, they are retrieved through a web service (googleusercontent.com). Any icons that require the user to be signed in will not work properly because it is redirected to the login page. I'm not sure why the Google Maps icon doesn't work though. You can follow https://bugzil.la/1315616 for progress on this issue.

Is it possible to add custom icons to get around that problem?