Back in the last century, supporting both browsers amounted to sniffing the browser and implementing the same feature twice. document.layers vs document.all for example.

Nowadays I think the problem is different: we just don't know what's going on. The site is transpiled from TypeScript, written on top of React or Vue which drastically switches paradigm (bonus for Tailwind), packed with building tools, and the average dev has little understanding of what actually comes out. It's a tall stack of leaky abstractions on top of the already tall one of the web. The dev is pretty sure it works on Chrome so they say it does work there, but it was not even a deliberate choice.

[–] [email protected] 5 points 4 days ago

For most sites it’s a testing matrix issue. Most testing teams look at browser stats and choose how to apply their limited resources based on that. So the dev probably doesn’t even see the bug that exists for an old Firefox version as there’s no testing done on it.