What you're viewing is the old default mobile template for mobile sites. I'm surprised that thing is not retired and removed already.
Scroll all the way down to the bottom of the page and hit the “full site” link at the end of the footer. It will instead take you to a mobile responsive version of the full site that uses all of its distinct site elements:
This feature is exclusive to the iOS and Android apps. From the announcing blog post:
At the heart of our mobile applications is The Feed which gives you a deeply personalized selection of content customized to what’s relevant to you.
It includes any new answers, comments, or upvotes on your posts, and even new questions we think you’d like based on ...
This has been fixed, although I’m not sure how. In any case, I am using the new request desktop switch (under website settings), which prevents it from reverting back at any point, even in incognito and also the browser in the SE app. (The only problem is that it needs to be done on each site and its meta, so I’m going to be doing this a lot, even if it was ...
Firefox for Mobile is not officially supported and never has been on our network. It has always been in a state of "it works but may break" in our list of supported browsers on Meta (which in fact still mentions version 10.0.3 from back in 2012).