It's partly that, but it's also partly that the pace at which new standards of useful things get implemented (pre-ratification and post ratification) entails patchy support plus differing implementation decisions from under-specified standards. My favorite example area right now is all the fun that is CSP. Firefox supports the SecurityPolicyViolation Event, which is useful so that you can let non-power users know about CSP issues without having to open the console, except it's gated by a setting in about:config that is off by default. Additionally for CSP's report-uri endpoint, Firefox won't send up any cookies with a request to that endpoint (https://bugzilla.mozilla.org/show_bug.cgi?id=1506992) which sucks if the endpoint relied on authentication that way. Chrome just works, meanwhile, on both of these issues. (Of course personally, if something doesn't work in Firefox, then I just don't use that something, or if I really need to (e.g. test business feature), I use chromium for just that one thing.)