caniuse and MDN compat data is really good these days, so you usually don't even need to crack open Firefox to be able to decide whether a JS API or a CSS feature is portable. For instance, in the Mega case, I assume they use the FileSystem API (since they have to download a large encrypted blob to a staging area first), and it's very easy to check availability of that API on all major browsers. As long as you don't chase the latest standards or WICG proposals, your code is likely going to work across all major browsers.[1] As I said in another comment, I occasionally check whether my sites work in Firefox and Safari, and they're just fine almost all of the time.
It seems to me that some diehard Firefox fans assume that just because some sites don't work on Firefox, all developers have to dedicate special attention to Firefox or their sites will be broken. That's very far from truth.
[1] More care is needed for mobile Safari, which often imposes additional non-obvious constraints, but that's a separate topic.