I'm on Firefox Mobile right now on my tablet, a Samsung Galaxy Tab Pro 8.4. The full site looks perfectly good with the only exception that "click targets" are annoyingly small.
My suggestion is that each user should be able to set his desired text size / DPI resolution per device (tracked via cookies of course), so that I can have properly clickable comment links and upvote buttons and so on on my tablet, and keep everything as before from my laptop.
Personally, I'd like the old mobile display back!!
Edit - I noticed that the OP was logged out in the screenshot, so I tried it again. Here is an image of the screen when I am not logged in:
http://imgur.com/xuhywK6
Here is the image of the screen when I am logged in:
http://imgur.com/kHjbFWz
The only difference between the two cases is that I was too lazy to log out, so I just pulled up the page in private mode.
I wish I had an iPhone 6 as I'd like to try this case with the OP's environment.