User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20100101 Firefox/17.0 Build ID: 20121130221520 Steps to reproduce: I opened some pages from various blogs in Nightly 21.0a1. Actual results: The 'book' icon that enables reader mode doesn't show up for any of the pages I tried. And I have not seen it for weeks on my casual browsing. I'm confident to generalize that it is not working for any webpage. Expected results: The 'book' icon that enables reader mode should show up for the pages I tested, both because it makes sense and because it does show up on Firefox Beta 18.0.

This is probably a dupe of bug 828124. What device are you using? Reader mode is now disabled for all devices that report less than 512 MiB of memory (this was done in bug 792603).

(In reply to Kartikaya Gupta (email:kats@mozilla.com) from comment #1) I use Samsung I9000 Galaxy S, with a custom ROM based on android 4.1 I agree, I'll mark it as a duplicate. Thanks for the detailed response.