User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:43.0) Gecko/20100101 Firefox/43.0 Build ID: 20160105164030 Steps to reproduce: Access various essays on the website criterion.com, a site that collects various films and publishes essays online. Some of the pages accessed include: • https://www.criterion.com/current/posts/3875-inside-llewyn-davis-the-sound-of-music • https://www.criterion.com/current/posts/3235-the-umbrellas-of-cherbourg-a-finite-forever • https://www.criterion.com/current/posts/483-army-of-shadows-out-of-the-shadows • https://www.criterion.com/current/posts/303-le-cercle-rouge-great-blasphemies • https://www.criterion.com/current/posts/2699-the-life-and-death-and-life-of-colonel-blimp Actual results: All of these sites work well with Reader Mode. By appending "about:reader?url=" to the beginning of each URL, Reader Mode loads each page perfectly fine. However, the default icon to enable reader mode that appears in the URL bar does not appear to toggle it on and off on the page. Expected results: The button that allows the user to enable and disable Reader Mode on certain pages should be present on the essay pages on criterion.com.

(In reply to nicolaasmverbeek+code from comment #0) > All of these sites work well with Reader Mode. By appending > "about:reader?url=" to the beginning of each URL, Reader Mode loads each > page perfectly fine. However, the default icon to enable reader mode that > appears in the URL bar does not appear to toggle it on and off on the page. Same result (no icon, but prepending "about:reader?url=" works) for me with current windows mozilla-central nightly. (Mozilla/5.0 (Windows NT 6.1; rv:47.0) Gecko/20100101 Firefox/47.0)