I thought I'd try this, and found that one thing it does is make it impossible to log into a site if you're using facebook to log into that site. When I clicked on the add-on icon I could get a list of sites that were allowed, and there was a section for sites *I* had allowed with nothing in it. And I could find no way to add a site to it. Not by clicking within the app icon. Not by going to manage add-on. And when I googled for help I found a page that said you can add sites but didn't explain how!
Had to remove it.