In Firefox mobile, there's usually an open book icon in the URL bar (at the right) that puts you in a sort of "reader mode", stripping away CSS and giving you text only. It's usually there for articles.
I had the same trouble with this article on the desktop. This happens a lot, designers assume everyone's running full screen, or are not on mobile, or just don't consider it. In desktop firefox I've installed addon Disable Style Button, which turns off CSS with one click. It's a shortcut for View/Page Style/No Style if you want to try it out without installing the addon.
The little book icon isn't always there, and anyway I generally like the result of turning off CSS anyway, it looks very nineties and I can just see the faintest outline of a middle finger.
> This happens a lot, designers assume everyone's running full screen, or are not on mobile, or just don't consider it
What kills me is that a basic 90's HTML 3 page can be zoomed and scrolled on mobile browser. If you don't care too much about fancy design (eg for personal sites), then just use HTML and really basic, or no, CSS. If you do care about sophisticated design, then you should ensure it works on the most common browsers and devices.
