In a related issue, it is also incorrectly detecting "private browsing" mode (false positive) in Firefox 27.0.1. I am using Greasemonkey (no scripts loaded for the relevant pages), HTTPS Everywhere, NoScript (all javascript whitelisted did not remedy this issue), and otherwise have all flash, javascript, and cookies enabled. The only thing I can think of is that NoScript could be blocking what it considers to be potential XSS attacks but usually I get an information bar about such things, and I received no alerts or notifications from NoScript or any other add-on.
Why is it even trying to detect private browsing / incognito mode? Just to maximize the benefit from ad revenue via tracking cookies? What a silly and annoying restriction, even if it was implemented correctly!