Doesn't work for pages with majority of text is under a spoiler. Similar page on the same website but with more text outside spoiler works just fine and reader mode shows text under spoler. In the case when it didn't work the amount of text outside spoiler was less but inside was more. Activating reader mode through about:reader?url= works, this addon is seemingly not much more useful than not having it.
What this add-on does is, it just helps activating the Reader View, which is an internal Firefox tool (see http://mzl.la/1cHarCP). Hence, requests concerning the Reader View itself (and not issues regarding opening it with this add-on) should be directed to Mozilla's Bugzilla platform and specifically tagged with "Component: Reader Mode": https://bugzilla.mozilla.org/buglist.cgi?component=Reader%20Mode&product=Toolkit&bug_status=__open__