Chrome - like IE before it - has a number of "features" that only work/ed in Chrome/IE.
Writing for a standards compliant browser like Firefox makes your code more well defined today just as it did back then because you won't get away with the same sloppyness. (This also makes you catch and fix problems in early iterations over the problem instead of after QA calls to complain so it saves you time and context switching too and if you are good you might look like a cross browser superhero almost for free ;-)
Earlier on not every basic thing was supported everywhere, many people here will remember the ACID tests. Younger devs won't remember them as we stopped talking about them after every browser became compliant. Today every mainstream browser has comprehensive test suites to cover everything we need from CSS I think.
Testing on ~~Chrome~~ Firefox would save you from accidentally using Chrome-only features, but that's only part of the problem, caniuse.com kinda works better for that as you get data about other browsers too.
>>Testing on Chrome would save you from accidentally using Chrome-only features, but that's only part of the problem,
>How?
Sorry I meant "Firefox" and "Chrome-only" there.
In general though it's not like Firefox is spec compliant and others aren't, pretty much every browser works a little different in some areas. Just for the sake of saying something verifiable: no browser is spec compliant because the spec mandates a precise maximum length for strings and each main browser has a different, arbitrary (= I have the RAM, they just won't let me use it), lower limit on that.
Writing for a standards compliant browser like Firefox makes your code more well defined today just as it did back then because you won't get away with the same sloppyness. (This also makes you catch and fix problems in early iterations over the problem instead of after QA calls to complain so it saves you time and context switching too and if you are good you might look like a cross browser superhero almost for free ;-)
Earlier on not every basic thing was supported everywhere, many people here will remember the ACID tests. Younger devs won't remember them as we stopped talking about them after every browser became compliant. Today every mainstream browser has comprehensive test suites to cover everything we need from CSS I think.