URL: https://www.google.com.ua/search?q=what+is+the+black+dot+Firefox+reading+view
Problem type: Something else
Description: In reader view a black dot appears on the screen
Steps to Reproduce It appears on every site.
Browser / Version: Firefox Mobile 56.0
Operating System: Android 5.1
Tested Another Browser: No

From webcompat.com with ❤️