Found a bug in the Avast online security addon (.1489) for Firefox.
Clicking on the "star" on a bookmarked page, Firefox will show a doorhanger to edit the preferences of the bookmark. On top there is a button that is normally called: "Remove Bookmark". With the addon enabled the button is empty, but still functional. Beside that, also the title above the button is gone: "Edit this Bookmark" and the Sync notification on the bottom. See screenshots below:
Showing the issue (addon enabled):
How it should have been (addon disabled):
