For me, the box seems to appear sporadically. It does not appear on Yahoo & Google new, but all the ads are indeed hidden. Does not seem to be a way to turn it off. It does appear on this Mozilla I am on now, and does toggle off, on. It is responding automatically to the webpage itself?

Note that not all pages support reader view, a website needs to meet certain conditions about paragraph length.

See also:

• https://support.mozilla.org/en-US/kb/firefox-reader-view-clutter-free-web-pages