Ironically, checking for UTC+0 JavaScript time from an obviously-not-UTC+0 IP geolocation might give additional hints for fingerprinting, as does everything else that looks non-default, like odd language preferences or even a DNT header.
This is a great example of why I'm generally skeptical of these scattershot approaches to making users more secure by changing default settings in mainstream browsers. Security and privacy features always entail tradeoffs and should be designed and implemented holistically for best results.
This is why I, a privacy-conscious individual, don't follow any of these guides in my Firefox. If you follow the discussion on Bugzilla, the weekly team meeting notes, and occasionally ask respectful questions on Mozilla IRC, you come to a similar conclusion to me in that the Firefox development community is doing the right thing in not enabling this by default.