After poking around a little bit, I discovered that this older Firefox v.37+ actually has a Responsive Design View mode in it’s Web Developer section. I activated it (only in the present tab) and must say that the website does not look broken in that mode:

So this whole thing is indeed a downward-compatibility problem. Perhaps, like with the reply default-post stating “This editor uses Markdown syntax” instructions, a note could be added about such a Responsive Design View mode in some browsers that don’t automatically switch to it. Just a thought.