Cannot find the option to NOT automatically mark options as read (Firefox Android 20)

I exported my settings from desktop thunderbird when setting up Thunderbird Android 20 but discovered it was automatically marking messages as read. That makes it difficult to have a peek at messages when I'm out (in case there's something urgent) but still see those messages as unread when I return to the desktop. https://support.mozilla.org/en-US/questions/1504592 claims to have found the option to do that, but it's an old message that has been archived without ever saying where that option was found. I've tried settings search as well as a visual search of all settings without seeing a relevant option. Does that option actually exist? Or do I have to go back to K9 (which, at least in v19, had that option)?