It's not unusual to visit a site (often something neglected but really important, like a support website), and find it simply doesn't load in Firefox.

Then I open it in Chrome and it works. And it's clear which browser the developers tested the site against.

When people come across such things please report them to https://webcompat.com/ There are some Firefox, IE and webdevs that work on understanding what is broken and trying to get sites to work cross browser.

Sometimes, it's clear what minimum window size they've assumed. I know of one (bank login) which doesn't work well on a netbook due to this.

I tried to report the problem via the site feedback link which they helpfully provide in most of their page footers. It fails in Firefox but works fine in Chromium; I ended up reporting two problems reported instead of one.

There have been extensions for Firefox that opened tabs in IE compatibility mode, which were needed occasionally in the past. I wonder if such a thing for chrome compatibility will end up having to be made. Hopefully not.

It's even more common for sites to require Chrome because they use a prefixed version of something that _is_ a standard, but they just can't be bothered to use the standard version...

Chrome doesn't use vendor prefixes for new features: http://www.chromium.org/blink#vendor-prefixes

Yes, I know. But they have plenty of old features that they support prefixed versions of, and people often enough use those without the corresponding standard syntax. This means, of course, that Chrome can't drop their prefixed versions without breaking websites. As a result, Edge and Firefox are now implementing a bunch of prefixed stuff simply to be web compatible.

For a view of what's needed in real-world terms for compat with the web nowadays, see https://compat.spec.whatwg.org/